U.S. officials say the Boston Marathon explosions that killed three people and wounded more than 260 were triggered by a remote-controlled detonator.
Two officials on Wednesday said the bombs were not very sophisticated. One of the officials described the detonator as 'close-controlled' — meaning it had to be triggered within several blocks of the bombs.
Both U.S. officials are close to the ongoing investigation but spoke on condition of anonymity because they were not authorized to discuss it publicly.
It was not immediately clear what the detonation device was, though the surviving bombing suspect, Dzhokhar Tsarnaev, was described in a criminal complaint as holding a cellphone minutes before the first explosion.
Cellphones have been used to trigger bombings in war zones.The suspected Boston Marathon bomber has admitted that he and his brother planted two bombs at the race - and had learned to make them through a notorious terror propaganda magazine.
Speaking to the FBI from his hospital bed in Boston, Dzhokhar Tsarnaev has also reportedly confessed that he and his brother Tamerlan planted the pressure cooker bombs, and were driven to commit the attack by the wars in Iraq and Afghanistan.
Tamerlan Tsarnaev was an ardent reader of jihadist websites and extremist propaganda, including Inspire magazine, an online publication produced by al Qaeda's Yemen affiliate.
The English-language magazine is dedicated to spreading al Qaeda's message, and even includes instructions on how to construct explosive devices.
In the Summer of 2010, an infamous article entitled 'Make a bomb in the kitchen of your mom' listed a pressure cooker bomb as the 'most effective weapon.'
The magazine has also endorsed lone-wolf terror attacks.
The method used to make the bomb is reminiscent of the failed attempt to bomb Times Square by Faisal Shahzad in 2010 - and other terror strikes in India, Afghanistan, Nepal and Pakistan.
Another Inspire article has insisted that a good time to 'inflict maximum human losses' is during a sporting event.
The article reads: 'There are numerous such targets such as crowded sports arenas, annual social events, large international exhibitions, crowded market-places, skyscrapers, crowded buildings.'
Directions for terror: In the Summer of 2010, an infamous article entitled 'Make a bomb in the kitchen of your Mom' listed a pressure cooker bomb as the 'most effective weapon'
Caught on tape: Dzhokhar Tsarnaev is seen on a store security camera just before the shootout with police that resulted in his brother Tamerlan's death
Kitchen bomb: The pressure cooker bomb is known as a 'highly effective' weapon of al Qaeda
Terror Attack: The two detonations went off near the finish line of the 117th Boston Marathon on April 15, 2013 - returning the specter of terrorism to the United States for the first time since 9/11
Hiding place: This image, taken from a surveillance camera shows Dzhokhar Tsarnaev climbing into a boat Friday morning after a police gun battle. He was later found in the boat and captured
Terror in Times Square: A pressure cooker bomb was one of the devices used in a foiled attempt to attack New York City in 2010
Shahzad was sentenced to life in prison in the Times Square attempt, which was foiled when a street vendor near Shahzad's explosives-laden SUV saw it smoking and flagged down police officers.
Similar pressure-cooker explosives have been used in Afghanistan, India, Nepal and Pakistan, according to a July 2010 joint FBI and Homeland Security intelligence report.
'Placed carefully, such devices provide little or no indication of an impending attack,' the report said.
Pressure cooker devices were also used in the 2006 train bombings in Mumbai, India, which killed 209 people and injured hundreds more.
Weapon: The method used to make the bomb is reminiscent of the failed attempt to bomb Times Square by Faisal Shazad in 2010 - and other terror strikes in India, Pakistan and Afghanistan
Meanwhile, Tsarnaev's condition was upgraded from 'serious' to 'fair,' as he recovers from wounds he sustained during a firefight with authorities in the early hours of Friday morning.
He was arraigned on Monday during a bedside court hearing, and has been speaking to the FBI.
The Washington Post reported that he confessed his role in the deadly twin bombing at the marathon finish line on April 15, and says that he and his brother were not acting under the direction of an overseas terror group.
Three people - Martin Richard, 8, Krystle Campbell, 29, and Lu Lingzi, 23 - were killed in the attack and more than 200 others were injured.
The manager of a New Hampshire fireworks store says the older of the two Boston bombing suspects bought two mortar kits at the store in February.
April Walton, of Phantom Fireworks, says Tamerlan Tsarnaev bought two 'Lock and Load' reloadable mortar kits with 24 shells each.
Company Vice President William Weimer says FBI agents visited the Seabrook store on Friday, interviewed staff and checked its computers.
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'My brother made me do it': Boston 'bomber' reveals in scrawled note that older sibling 'wanted to defend Islam from attack' - and was motivated by US wars in Iraq and Afghanistan
Bombing suspect Tamerlan Tsarnaev 'called his wife a slut and a prostitute' friends reveal as feds question her over marathon attacks
Bombing suspect Tamerlan Tsarnaev 'called his wife a slut and a prostitute' friends reveal as feds question her over marathon attacks
Boston bombers' last stand: Incredible new pictures emerge of the dramatic street shootout with police that left one brother dead
Bomber's ex-GIRLFRIEND revealed: High school sweetheart 'slapped' by Tamerlan calls him a 'bully' and a 'little tough guy'
He says the amount of gunpowder that could be harvested from the kits would not have been enough to detonate the Boston bombs.
Walton says Tsarnaev paid $200 cash but scanned his driver's license into the company's computer system as required by store policy.
Walton says the employee who handled the sale described it as a routine transaction.
Boston police scour Dzhokhar Tsarnaev's hideout boat
Suspects: Dzhokhar, right, and Tamerlan Tsarnaev are accused of planting the Boston bombs last Monday
Shielded: Katherine Russell, the widow of Boston Marathon bombing suspect Tamerlan Tsarnaev, leaves her lawyer's office on Tuesday
While Dzhokhar appears to be cooperating, investigators have little information about his 26-year-old brother, who died in a gunfight with police leaving no explanation for his crimes.
But a picture has emerged in the days since Tamerlan was killed of a proud but angry young man who never quite achieved his own idea of the American dream, but found solace instead in a radical form of Islam adopted by fighters in his homeland.
What now seems clear is that he was deeply influenced by a few months' sojourn in Makhachkala, Muslim Dagestan's capital on the Caspian Sea, where children in the street play 'cops and guerrillas' and bombings and shootings are everyday news.
Their father, Anzor Tsarnaev, had held high-status government jobs in the central Asian former Soviet state of Kyrgyzstan, but eventually found work as a mechanic in Massachusetts.
Their mother, Zubeidat Tsarnayeva, increasingly turned to religion.
American dream: Father Anzor, left, and mother Zubeidat, center, with baby Tamerlan and an uncle sought a better life in the U.S.
Separate ways: The couple later divorced and split their time between Dagestan and Boston
The parents eventually split up and moved back to Dagestan in southern Russia, although their mother was in the United States as recently as last June, when she was arrested by police in Natick, Massachusetts and charged with stealing women's clothing valued at $1,624 from a Lord & Taylor store.
She was also charged with two counts of malicious damage to property after allegedly trying to remove the anti-theft sensors on the clothing, said Lieutenant Brian Grassey of the Natick police.
He had no information on the outcome of the case.
The brothers attended Cambridge Rindge and Latin High School, a public school that counts actors Matt Damon and Ben Affleck among its alumni but has a diverse student body that includes working-class immigrants from around the world.
Dzhokhar may have been influenced by his charismatic older brother.
A video posted on the Internet by the younger Tsarnaev with the caption 'My brother, seriously messing around' shows Tamerlan as a mesmerizing goofball, sending up Chechens, Dagestanis and other ex-Soviet minorities with comedy accents in Russian, while the cameraman, presumably Dzhokhar, laughs wildly in admiration.
Working alone: Dzhokhar Tsarnaev, left, may have been influenced by his older brother Tamerlan, right, pictured at the Boston Marathon
But it was the charming older Tsarnaev brother who seemed to have a harder time fitting in.
Through high school, and for years afterward, Luis Vasquez thought of his classmate Tamerlan Tsarnaev as 'a big, friendly giant', who never lost the accent in his slow, deep voice.
The morning after Tamerlan was killed in the shootout, news reports seized on a quote from a caption in a photo essay that showed him training for boxing at a gym, in which he said he had no American friends because no one there understood him.
Mr Vasquez said the quote has been misinterpreted: Tamerlan may have preferred the company of other immigrants and minorities to white, native-born Americans, but he was no loner.
The pictures clearly show him as a cocky, sociable young athlete, confident of his sporting skills, dressing in smart clothes and play-boxing with an attractive blonde woman.
He was planning to represent New England at a national competition.
Tsarnaev was a bit arrogant when it came to his boxing, 'but what athlete isn't cocky?' Vasquez said.
'He knew he was good. He was good. So that's OK. No one's going to challenge that.' But about three years ago, Tsnarnaev abruptly dropped off the scene.
He removed his Facebook page. Vasquez no longer saw him around the streets. 'He kind of disconnected himself,' Vasquez said.
He asked mutual friends if they had seen Tsarnaev, if they knew what he was up to.
Some said they had heard he went back to Russia. But no one had details. No one knew he had married or had a child.
Studies: Tsarnaev went from boxing and college to a life of unemployment and religion
The American dream never arrived for Tamerlan Tsarnaev. His boxing career didn't work out, nor did his studies at junior college.
He was briefly detained by police when a girlfriend complained of violence, but the accusations were dropped.
He married Katherine Russell, a student at Suffolk University from a middle-class Rhode Island background, the daughter of a doctor and a nurse.
Katherine converted to Islam and left the university without graduating.
They had a daughter, Zahara, now 3. Tamerlan seems never to have found a full-time career. His family members say his new wife supported him, while he stayed home with the child.
Mary Silberman, whose apartment backs up to the rooms Tamerlan Tsarnaev rented, said she often heard loud arguments late at night - a woman's voice, mostly, yelling at a man.
Married: Tamerlan was often heard by neighbors arguing with his wife and mother of his daughter Katherine Russell
'It wasn't enough to call the police,' Silberman said. 'It didn't sound like anyone was in physical danger.'
Yet the shouting, erupting at 11pm or midnight, was loud enough to keep her up.
Silberman also heard the couple's young daughter crying frequently at night, the wails so sharp that she wondered why the mother wasn't responding.
As the girl grew into a toddler, Silberman would see her sometimes staring out the window.
Their bedroom window had no proper curtains, just two mismatched sheets hung haphazardly to block the light. Plastic bags were stuffed in the sides of the window, apparently to keep out the cold.
'It wasn't what you would think of as normal,' Silberman said. The family made her so uncomfortable, she said, that she made sure she pulled her chair out of view of their windows when she sat on her back deck.
The Russell family issued a statement the morning after the shootout saying they realized they had never really known Tamerlan Tsarnaev.
The family home in leafy North Kingston Rhode Island is on the market for $467,000, and police cars now patrol the street.
Katherine Russell did not respond to requests for comment via her lawyer.
'Crude bombs': Both of the bombs were small, likely homemade devices and initial tests showed no C-4 or other high-grade explosive material