The elder Zazi is charged with hiding evidence in the case against his son in an obstruction of justice case headed toward trial later this year. And as part of a plea deal, the government can require his jailed son to testify at that trial or that of Adis Medunjanin, an alleged accomplice in the plot to blow up the subways with homemade backpack bombs.
Prosecutors have declined to discuss potential witnesses.
But the father's lawyers say they believe that besides his notorious son, at least three lesser-known members of the Zazi clan in Colorado have betrayed their client, Mohammed Wali Zazi, by secretly becoming government cooperators.
The defense wants to see any agreements promising the three "assistance ... in obtaining leniency in any court, or lack of prosecution or arrest, or any other favorable treatment," the lawyers wrote last week in a letter to federal prosecutors in Brooklyn.
The relatives - the elder Zazi's sister, nephew and brother-in-law - have not been charged with any crimes. But, as in most terror probes, it's likely investigators scoured immigration and other records of anyone close to Najibullah Zazi for alleged violations that could be used as leverage against them.
How the relatives became possible cooperators "could become clearer at trial," said Deborah Colson, the elder Zazi's lawyer. A U.S. attorney's spokesman declined comment on Wednesday.
Najibullah Zazi pleaded guilty on Feb. 22 and is cooperating in an ongoing investigation of the subway plot and its roots in Pakistan, where Zazi said he went with former high school friends - Medunjanin and Zarein Ahmedzay - in 2008 to seek terror training from al-Qaida.
Najibullah Zazi, an airport van driver from Colorado, admitted that once back from Pakistan he tested peroxide-based explosive materials in a makeshift lab in Denver in the fall of 2009 before traveling by car to New York to carry out the scheme.
Authorities say Medunjanin and Ahmedzay agreed to join Zazi in three coordinated suicide bombings on Manhattan lines during rush hour near the eighth anniversary of the Sept. 11 terror attacks - what Zazi called a "martyrdom operation."
"I would sacrifice myself to bring attention to what the U.S. military was doing to civilians in Afghanistan," said Zazi, who was born in Afghanistan before his family moved to Pakistan.
The plot was disrupted when police stopped Zazi's car as it entered New York.
Like Zazi, Ahmedzay has pleaded guilty and is cooperating. Both men could end up testifying against Medunjanin, who is fighting the charges.
Zazi faces a life term when he is sentenced. His sentencing date is June 24. But the date has been reset a number of times. There was no immediate response Wednesday to messages left with his lawyer.
U.S. prosecutors still are seeking the extradition of another suspect in Britain they say has ties to the scheme.
Mohammed Wali Zazi was charged in October 2009 with conspiring to destroy or hide "glasses, masks, liquid chemicals and containers" that were evidence in the case against his son. At the time, the 55-year-old Afghan immigrant and former New York City taxi driver appeared to be a minor player and likely candidate for a plea deal.
But since then - and despite his son's plea - federal prosecutors have instead taken a harder line: A new indictment filed late last year added additional obstruction counts against the father, along with a charge alleging he lied when he denied having a telephone conversation about his son. The indictment also alleges he supplied false information for an asylum application for his nephew, Amanullah Zazi.
The nephew, along with his mother, Rabia Zazi and her husband, Naqip Jaji, were the three named as possible witnesses in the defense papers.
Rabia Zazi and Jaji were widely quoted in early press accounts about the investigation. Outside her suburban Denver home in September 2009, the aunt downplayed suspicions Najibullah Zazi was involved in terrorism, saying, "He doesn't have time. He's working."
Efforts to contact the three relatives through a family lawyer in Colorado were unsuccessful.
Mohammed Wali Zazi remains free on bail in Colorado, where he once ran a small limo service. His lawyer said he's allowed to work while awaiting trial in July, but can't find a job.
"It's been very difficult," she said.
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Associated Press Writer P. Solomon Banda in Denver contributed to this report.
