One man's quest to explain his brother's mysterious jail cell death 19 years ago has rekindled long-dormant questions about whether others were involved in the 1995 Oklahoma City bombing.

What some consider a far-fetched conspiracy theory will be at the forefront during a trial set to begin on Monday in Salt Lake City. The Freedom of Information Act lawsuit was brought by a Salt Lake City attorney, Jesse Trentadue, against the FBI. He says the agency will not release security camera videos that show that a second person was with Timothy McVeigh when he parked a truck outside the Oklahoma City federal building and detonated a bomb, killing 168 people. The government claims McVeigh was alone.

Unsatisfied by the FBI's previous explanations, US district judge Clark Waddoups has ordered the agency to explain why it cannot find videos from the bombing that are mentioned in evidence logs, citing the public importance of the tapes.

Trentadue believes the presence of a second suspect in the truck explains why his brother, Kenneth Trentadue, was flown to Oklahoma several months after the bombing, where he died in a federal holding cell in what was labelled a suicide. His brother bore a striking resemblance to the police sketch that officials sent out after the bombing based on witness descriptions of the enigmatic suspect "John Doe No2", who was the same height, build and complexion. The suspect was never identified.

"I did not start out to solve the Oklahoma City bombing, I started out for justice for my brother's murder," Jesse Trentadue said. "But along the way, every path I took, every lead I got, took me to the bombing."

The FBI says it cannot find anything to suggest the videos exist, and says it would be "unreasonably burdensome" to do a search that would take a single staff person more than 18 months to conduct.

Jesse Trentadue's belief that the tapes exists stems from a secret service document written shortly after the bombing that describes security video footage of the attack that shows suspects – in plural – exiting the truck three minutes before it went off.

A secret service agent testified in 2004 that the log does, in fact, exist but that the government knows of no videotape. The log that the information was pulled from contained reports that were never verified, said Stacy A Bauerschmidt, then-assistant to the special agent in charge of the agency's intelligence division.

Several investigators and prosecutors who worked the case told the Associated Press in 2004 they had never seen video footage like that described in the Secret Service log.

The FBI has released 30 video recordings to Trentadue from downtown Oklahoma City, but those recordings do not show the explosion or McVeigh's arrival in a rental truck.

If he wins at trial, Trentadue hopes to be able to search for the tapes himself rather than having to accept the FBI's answer that they don't exist.

Kathy Sanders and Jannie Coverdale, who both lost grandchildren in the bombing, are grateful for Trentadue's pursuit of the case. Sanders said she has been waiting 19 years to see the tapes.

"It is worth pursuing," Coverdale said. "I know there was somebody else. I have never stopped asking questions."

But former Oklahoma Representative Susan Winchester, whose sister, Dr Margaret Clark, was killed in the bombing, said she is satisfied that officials have identified everyone responsible for the bombing.

"I was very comfortable with the decisions that came out of the federal and state trials," Winchester said. "I have reached that point in my life where I can continue."

Jesse Trentadue's mission began four months after the bombing when his brother died at the US Bureau of Prisons' Federal Transfer Centre in Oklahoma City. Kenneth Trentadue, 44, a convicted bank robber and construction worker, was brought there after being picked up for probation violations while coming back to the US at the Mexican border, Jesse Trentadue said.

His death was officially labelled a suicide. But his body had 41 wounds and bruises that his brother believes were the result of a beating. In 2008, a federal judge awarded the family $1.1m (£650,000) in damages for extreme emotional distress in the government's handling of the death, but the amount was reduced to $900,000 after an appeal.

Jesse Trentadue's best guess about the motive is that his brother died in an interrogation gone wrong by investigators demanding information Kenneth Trentadue did not have.

Jesse Trentadue filed the Freedom of Information Act lawsuit in 2008.

Going toe to toe with the federal government has come at a personal price for Jesse Trentadue, 67, who says he has lost time with his children and wife that he cannot recover.

But he has no regrets, fuelled by his love for his brother. Just three years apart, the two shared a bed, hunted raccoons together and played on the same sports teams growing up in a coal camp in West Virginia.

Their paths diverged as adults – Jesse becoming an attorney while Kenneth fell into drugs and crime – but the brotherly bond never broke. Before his death, Kenneth Trentadue had overcome his heroin addiction and had a newborn baby at home in San Diego, Jesse Trentadue said. The brothers spoke by phone from jail the night before his death, with the two discussing how he would soon be out.

"What I learned growing up in the coalfields is that you fight even when you know you can't win," he said. "Because you have to make a stand on some things. Justice for my brother is certainly one of them."