Najibullah Zazi

An FBI vehicle picks up Najibullah Zazi at his Aurora apartment. (September 18, 2009)

DENVER, Colo. - Law enforcement sources tell FOX News that Najibullah Zazi, the Aurora, Colorado man under investigation in a suspected bomb plot targeting New York subways, has admitted to having involvement with the al-Qaida terrorist network.

Zazi, 24, was questioned by the FBI on Friday for the third straight day following Wednesday's search of his apartment on East Smoky Hill Rd. near E-470 and a home belonging to his aunt, also in Aurora.

FOX News reports, citing Department of Justice sources, that Zazi is negotiating a plea agreement on unspecified terrorist-related charges.

After Zazi traveled to New York City last weekend, FBI agents and police officers armed with search warrants searched three apartments in Queens. It was not immediately known what items were found during those raids, or the searches in Colorado.

Additionally, source say federal agents visited a Home Depot location somewhere in the Denver area Monday to inspect receipts for purchases of muriatic acid, also known as hydrochloric acid. That chemical is normally used to clean concrete and treat swimming pools. However, it can also be used in the manufacturing of explosives.

Zazi's attorney, Arthur Folsom, has said his client never met with al-Qaida operatives and isn't involved in terrorism.

"He's simply somebody who was in the wrong place at the wrong time," Folsom said Thursday. Zazi himself also proclaimed his innocence to reporters Tuesday night.

"I have nothing to do with al-Qaida...any link or any thing with al-Qaida," Zazi told FOX 31's Eli Stokols.

However, after back-to-back 8-hour interrogations at the Denver FBI offices on Wednesday and Thursday, Zazi told law enforcement a different story Friday, FOX News reports.

Zazi, who unnamed authorities have said trained at a Pakistani terror camp, reportedly had bomb-making diagrams on a computer that he carried with him on his visit to New York. The laptop was in Zazi's car as he drove from Colorado to New York City, arriving the day before the 8th anniversary of the Sept. 11, 2001 terrorist attacks.

U.S. Attorney General Eric Holder insisted Friday there was no immediate danger.

"There are no imminent threats, on the basis of what we have uncovered," Holder told reporters in Minneapolis. "The FBI is working this case around the clock in both cities and in other parts of the country. And we will make sure that if there are crimes that were committed that they will be charged and people will be held accountable."

A joint FBI-New York Police Department task force feared Zazi may be involved in a potential plot involving hydrogen peroxide-based explosives like those cited in an intelligence warning issued Monday, said two other law enforcement officials, who spoke on condition of anonymity because they were not authorized to speak about the investigation.

Federal intelligence officers have sent state and local officials details about different hydrogen peroxide-based explosives. Three separate memos, dated Sept. 16 and obtained by The Associated Press, include descriptions about how to identify specific combinations of explosives: hexamethylene triperoxide diamine, hydrogen peroxide and organic fuel mixtures, and triacetone triperoxide. Each memo states that these explosives could be used in "an attack against the United States" and can be made at home.

Folsom said Zazi, 24, was born in Afghanistan in 1985, moved to Pakistan at age 7 and emigrated to the United States in 1999. Zazi's aunt had said earlier that he was born in Pakistan and grew up in Queens, N.Y.

Folsom said Zazi has returned to Pakistan four times in recent years: in 2004 because his grandfather was sick and dying, in 2006 to get married, and in 2007 and 2008 to visit his wife.