Updated

Reports that a second terror bombing may have been planned for the anniversary of the Oklahoma City bombing made Terry Nichols' (search) "blood boil," the convicted bomber said in an exclusive letter sent to FOX News' Rita Cosby.

"The truth is those claims by that certain inmate are pure lies and total fabrications," Nichols wrote. "I never said those things ever to anyone at anytime!"

Three weeks ago, FBI agents searched Nichols' former home and found blasting caps and other explosive materials apparently related to the 1995 attack, after they were tipped they may have missed evidence a decade ago.

The FBI had initially dismissed the tip that Nichols hid explosives — which warned the materials might be used for a bomb plot coinciding with the 10th anniversary of the worst domestic terror attack in U.S. history.

The FBI found no evidence that an attack was in the works for the April 19 anniversary, but the information that explosives had been hidden in Nichols' former home in Herington, Kan., turned out to be true.

The tip came from imprisoned mobster Gregory Scarpa Jr. (search), 53. Scarpa is an inmate in the same maximum-security federal prison in Florence, Colo., where Nichols is serving life sentences for his role in the attack on the Alfred P. Murrah Federal Building (search) that killed 168 people.

"It made my blood boil when I heard my attorney read that a certain individual here at the ADX claim[s] that I told him that there would be a 2nd bombing which would occur on the 10th anniversary of the OKC bombing," Nichols wrote.

Scarpa says he learned about the explosives from Nichols, mainly through notes passed between them, according to Stephen Dresch, a Michigan man who is Scarpa's informal advocate.

Dresch says he gave the information to the FBI in early March, but FBI agents did not search the vacant house until March 31.

The bureau did not act more quickly because Scarpa failed a lie detector test, a law enforcement official, who declined to be identified because of the sensitivity of the investigation, told The Associated Press.

An FBI lab is still examining the bomb-making materials for fingerprints and other clues that might show where the explosives originated and who may have had them before they got into Nichols' home.

Scarpa, a member of the Colombo organized crime family serving 50-plus years on drug trafficking, conspiracy and racketeering convictions, first communicated information about the explosives on March 1, then provided more details on March 10 and 11, Dresch said in letters sent to the staffs of two members of Congress and to the FBI's Detroit office. Scarpa revealed the location of the house on March 11, Dresch said.

"This inmate created those lies & others and weaved them in with the truth so he could make himself look as good & as important as possible to authorities so he could get as sweet of a deal as possible on a reduction in his prison sentence," Nichols wrote.

Nichols said he was reluctant at first to speak out on the anniversary of the bombing, but felt the strong need "to help set the record straight ... so that the people of Oklahoma can ease their concern & fears of such a thing occurring or supposedly planned to occur.

"It's a time for the victims and survivors, not me," Nichols wrote. "I did not want anyone to feel that I was wanting to, in any way, take away their special time. I did not want to be in the spotlight at all for it would stir up some angry and ill feelings in many."

Dresch's first letter said Scarpa learned from another prisoner, assumed by Dresch to be Nichols, "the location of a bomb on U.S. soil." The second described two rock piles in the crawl space beneath Nichols' former home. Under one, it said, were cardboard boxes wrapped in plastic. Those details match what the FBI said it found.

Aides to Reps. William Delahunt, D-Mass., and Dana Rohrabacher, R-Calif., acknowledged receiving the letters by fax. Delahunt's office received the letter on March 1 or 2 and forwarded it to the FBI, said Steve Schwadron, the lawmaker's chief of staff. The letter to Rohrabacher was not read until after the FBI search had been done, Rohrabacher spokeswoman Rebecca Rudman said.

Delahunt has chided the FBI for its dealings with informants, while Rohrabacher is considering requesting a hearing on the bureau's handling of the Oklahoma City investigation. Timothy McVeigh was convicted of federal conspiracy and murder charges in the bombing and executed in 2001.

"I'm more concerned that the FBI didn't do a thorough job investigating this location 10 years ago than I am about how long it took to follow through on an informant's tip," Rohrabacher said.

Dresch, a Michigan economist, principal owner of Forensic Intelligence International and former state lawmaker, speculated that the FBI didn't act more quickly because Scarpa has a long, contentious history with federal authorities.

Valerie Caproni, now the FBI general counsel, was a prosecutor in Scarpa's 1998 trial in Brooklyn, N.Y. At the time, Scarpa testified he spied for the FBI on four suspects in the 1993 World Trade Center bombing, including convicted mastermind Ramzi Yousef, while they were jailed together in Manhattan.

Scarpa said he passed on to the FBI plans that said associates of the four men would kill a prosecutor in one of Yousef's trials and attack a federal judge he declined to name as well as unspecified "government installations."

Caproni and U.S. District Judge Reena Raggi scoffed at Scarpa's claims, which the judge called insignificant at best and more likely "part of a scam."

Freelance journalist Peter Lance has argued in his recent book, "Cover Up," that Scarpa's information was accurate and included tips that could have led the FBI to Khalid Shaikh Mohammed, Yousef's uncle, long before the Sept. 11 attacks Mohammed purportedly helped plan.

FOX News' Rita Cosby, Andrew Hard and The Associated Press contributed to this report.