DENVER -- Mohammed Zazi, whose son is the key suspect in an international terror probe, pleaded not guilty in Denver federal court Friday to lying to investigators about the suspected bomb plot.

The father, a 53-year-old Aurora airport shuttle driver, was indicted by a Denver grand jury Thursday for making false statements to the FBI last month.

Zazi allegedly denied speaking on the phone with someone about his son being in "trouble" with authorities. What he didn't know: the FBI had secretly recorded his 20-minute conversation on Sept. 11 with a New York Muslim cleric. That cleric had told the father that New York police were asking questions about his sonl, the FBI said.

The elder Zazi's public defender, Edward Harris, vowed to challenge the legality of the FBI's wire-tapping of the father's phone conversations. Harris said the Afghani-immigrant's poor grasp of English could also play a key role in the case.

During the brief hearing, U.S. Magistrate Craig B. Shaffer read the indictment and explained Zazi's legal rights and potential 8-year prison penalty as a Pashtun interpreter translated for the defendant.

Harris entered the not guilty plea on his client's behalf, as Zazi, clad in a black pinstriped suit and open-collared white shirt, sat quietly listening.

Twice the interpreter said, "Yes," for Zazi when the judge asked if he wanted to confer privately with his attorney and when the judge asked if Zazi wanted to keep his court-appointed counsel.

Outside the court, Harris went on the offensive.

"To no great surprise, Mr. Zazi was indicted," he said. "I say to no great surprise because it takes a very minimum quantum of evidence to get an indictment in a case."

"We intend ... to vigorously represent him," the attorney said.

Asked if Zazi's language barrier and the "materiality" -- or relevance -- of the evidence would be key to his defense, Harris said, "We've not seen a shred of evidence yet. But I think it's safe to say that both language and materiality are likely to be important issues in this case."

Harris also said he would challenge whether the FBI had legal authority to intercept Mohammed Zazi's phone conversations under the sweeping power of the Foreign Intelligence Surveillance Act, known as FISA. The defense has not heard or seen the evidence, because eavesdropping evidence collected under FISA has a top-secret classification, requiring defense attorneys to get clearance to review evidence.

"We do plan to challenge the so-called FISA evidence in the case," Harris said.

In court papers, Harris criticized federal authorities' history of using questionable statements to obtain wiretaps from the special FISA court.

"Counsel asks the court to note the government's track record of submitting FISA applications with intentionally or recklessly false statements or material omissions," the attorney wrote. "Indeed, in at least 75 instances, it has confessed error relating to 'misstatements and omissions of material facts' that it had made in its FISA applications."

The elder Zazi remains free on $50,000 bond and electronic monitoring.

Zazi has a tentative trial date set for Dec. 7.

His son, who was arrested with the father in Denver Sept. 19, sits in a New York jail, charged with conspiring with unidentified accomplices to unleash weapons of mass destruction against the United States.

Prosecutors accused Najibullah Zazi of traveling to an al-Qaida training camp in Pakistan in 2008 where he allegedly received bomb-making training and instructions. The FBI said it found a recipe for crafting homemade bombs from beauty product chemicals on the young Zazi's laptop computer.

The younger Zazi and unidentified men are also accused of buying large quantities of the chemicals at Aurora beauty supply shops from July to September. Zazi was captured on store security cameras making the purchases, according to court records.