Crime historian digs for evidence in D.B. Cooper cold case on banks of Columbia River near where boy found $6,000 of the skyjacker's ransom money in 1980

A crime historian hoping to crack the infamous D.B. Cooper skyjacking cold case has conducted a dig for new evidence on the banks of the Columbia River in Washington state. 

Eric Ulis, 55, and four volunteers launched their excavation efforts on Friday, 50 years after D.B. Cooper vanished out the back of a Boeing 727 into freezing rain wearing a business suit, a parachute and carrying a pack with $200,000 in cash. 

They suspended the search without any significant developments on Saturday afternoon, but Ulis told DailyMail.com he's planning to go back to the site later this month.  

Eric Ulis, 55, and a team of four volunteers used hand shovels and other small-scale tools

The search took place along the banks of the Columbia River in Vancouver, Washington

FBI composite sketches of D.B. Cooper, who hijacked a Northwest Orient Airlines plane in November 1974 and jumped out over the southwestern portion of Washington with $200K

Ulis, a self-described expert on the infamous D.B. Cooper case, said he and his team exposed eight or nine square feet along the shoreline by digging two feet deep using hand shovels and iron bars.  

'It'll probably have to be within the month that I’ll be up here again,' he said. 'To really uncover what is about 300 sq ft of beach that is essentially locked in time.

'All this debris, all this rock, it’s almost like concrete to a certain degree.'

They were searching for evidence about 10 to 15 yards away from where a boy found $6,000 of Cooper's ransom money in 1980.

'It’s one area the the FBI never searched,' Ulis said.

Ulis says he expects to return to the site later this month after clearing just eight to nine sq ft in his search for Cooper's belongings or money, some of which was found nearby in 1980

On November 24, 1971, the night before Thanksgiving, a man described as being in his mid-40s with dark sunglasses and an olive complexion boarded a flight from Portland, Oregon, to Seattle-Tacoma International Airport. 

He bought his $20 ticket under the name 'Dan Cooper,' but an early wire-service report misidentified him as 'D.B. Cooper,' and the name stuck.

Sitting in the rear of the plane, he handed a note to a flight attendant after takeoff. 

'Miss, I have a bomb and would like you to sit by me,' it said.

The man demanded $200,000 in cash plus four parachutes. 

He received them at Sea-Tac, where he released the 36 passengers and two of the flight attendants. 

The plane took off again at his direction, heading slowly to Reno, Nevada, at the low height of 10,000 feet. Somewhere, apparently over southwestern Washington, Cooper lowered the aircraft's rear stairs and jumped.

He was never found, but a boy digging on a Columbia River beach in 1980 discovered three bundles of weathered $20 bills - nearly $6,000 in all. It was Cooper's cash, according to the serial numbers.

The Northwest Airlines jetliner sat on a runway for refueling at the Seattle airport. Cooper released the passengers there, took the $200K he demanded, and headed for Reno, Nevada

Ulis said his theory is that Cooper buried the parachutes, an attache case and the money at the same time, but dug smaller holes instead of one large one.

His search is taking place in private property with permission from the owner. He says he's using a small team and handheld tools in the largely agricultural area so as to not run afoul of state environmental regulations..

The case of Cooper has become infamous, not only in the Pacific Northwest but also in the country. 

The FBI Seattle field office called the investigation one of the longest and most exhaustive in the agency's history. 

It's the only unsolved skyjacking case in US history.

Over the years, the FBI and amateur sleuths have examined innumerable theories about Cooper's identity and fate, from accounts of unexplained wealth to purported discoveries of his parachute to potential matches of the agency's composite sketch of the suspect.

Sheridan Paterson, a man long suspected by the FBI to be the real 'Cooper,' died in January

Sheridan Paterson, a man suspected by the FBI to be the real 'Cooper,' died in January in Northern California, possibly taking the truth about the case to his grave. 

Paterson was a risk-taker who loved skydiving. He spent time in the Marines and working for aerospace giant Boeing. 

FBI agents showed up to interview Peterson's ex-wife at her high school counseling office in Bakersfield, California just weeks after the November 1974 hijacking. 

Asked if her ex-husband could be DB Cooper, she replied: 'Yes, that sounded like something he'd do.' 

But Paterson maintained he was living in a mud hut in Nepal working on a 'protest novel' about his experiences in Vietnam at the time of the incident.

In July 2016, the FBI announced it was no longer investigating the case.