The admission from Timothy McVeigh's convicted co-conspirator came to light when a copy of a 2003 "proffer," or a statement that Nichols drafted with his lawyers, was leaked to the Oklahoma City newspaper. Proffers detail to prosecutors what a suspect will testify to in return for a hoped-for plea bargain.

In the document, Nichols admits that he accompanied McVeigh when the Gulf War veteran purchased most of the materials used to construct the fertilizer bomb that destroyed the Alfred P. Murrah Federal Building. Nichols also confessed that he had personally helped McVeigh build the bomb in the back of a Ryder truck the day before the attack.

Nichols was convicted of conspiracy and the involuntary manslaughter of eight federal employees in federal court in 1997 and sentenced to life without parole. Six years later, state prosecutors in Oklahoma charged him with capital murder in the attack's other 160 fatalities.

Eager to escape the fate of McVeigh, who was tried separately and executed in June 2001, Nichols sought to persuade prosecutors to spare his life by offering them a written admission that included previously unknown details of the bombing.

The prosecutors were unmoved, noting that Nichols still refused to reveal the hidden location of leftover bomb components.

Nichols was convicted, but the jury deadlocked during the penalty phase. He was sentenced to a second life term without parole and returned to federal custody.