Rob Nigh, a defense attorney who represented Oklahoma City bomber Timothy McVeigh and witnessed his 2001 execution, died Sunday after a battle with cancer, former colleagues said. He was 57.

Known for his encyclopedic legal knowledge, work ethic and intense preparation on every case he handled, Nigh also defended those accused of committing some of the state's most egregious crimes. Nigh died about three months after stepping down as Tulsa County's chief public defender for serious health issues.

Stephen Jones, McVeigh's lead defense attorney, told The Associated Press Sunday that he asked Nigh to be his assistant on the McVeigh case and recommended him to serve as the former soldier's appellate attorney because he thought of his colleague as "a zealous advocate" for clients.

Jones said his instinct to take Nigh on didn't disappoint.

"He was methodical," Jones recalled. "He prepared the basic outline of the defense of Mr. McVeigh, which was about 50 pages. He cross-examined many of the witnesses that were at trial, and I entrusted a great amount of responsibility to him."

After joining the McVeigh defense team, Jones noticed that Nigh had forged a unique relationship with McVeigh, who was convicted of the 1995 bombing of an Oklahoma City federal building that killed 168 people and was considered to be the worst act of terrorism carried out on U.S. soil before the 9/11 terror attacks.

"He wouldn't be afraid to say he was Tim's friend," Jones said. "Rob considered it an honor to be asked by Tim to witness his execution ... Tim's cremains were entrusted to Rob."

In a statement Nigh released after McVeigh's execution in 2001, Nigh wrote that McVeigh's case "must make us realize that we are too fallible, we are simply too human to extract so final and irreversible a punishment."

Beyond the McVeigh case, Nigh represented defendants accused of committing some of Oklahoma's worst crimes, believing that every person was entitled to a defense, said colleague Cheryl Ramsey.

"He was the champion of the people who needed representation," Ramsey said. "He was honest, he was sincere and he was trusted by everyone."

Nigh and Ramsey each represented one of two then-teenage brothers accused of fatally stabbing their parents and three siblings in in their suburban Tulsa home in 2015.

In defending Michael Bever, who was 16 at the time of the killings, Nigh argued — unsuccessfully — that Bever be tried as a juvenile, asserting that state juvenile code allowed courts several options to balance a child's rehabilitation, along with protecting the public.

"He's a child," Nigh told the AP after a 2015 hearing. "The reality of the situation is we should not have children going to the state penitentiary."

Nigh also defended two men convicted of randomly killing three black residents in Tulsa over Easter weekend in 2012 and a man accused of fatally shooting his Lebanese neighbor in 2016 because of his race.