AURORA, Colorado (CNN) -- One of three men charged with lying to federal agents during a terrorism probe warned a co-defendant that their phone call was being monitored, according to court papers.

Najibullah Zazi is accused of making false statements to officials about an alleged bomb plot in the United States.

FBI agents included what they said was a transcript of the intercepted call in affidavits supporting the charges against Najibullah Zazi, his father Mohammed Wali Zazi and Ahmad Wais Afzali. All three face court appearances Monday in a case that the Justice Department said stretches from Colorado and New York to Pakistan.

"I was exposed to something yesterday from the authorities," Afzali told Najibullah Zazi in a September 11 call. "And they came to ask me about your characters. They asked me about you guys ... I told them that they are innocent, law-abiding."

Afzali said, however, called the questions from police "a good sign."

"The bad sign is for them coming to you guys and picking you up automatically," he said. And he told the younger Zazi, "Don't get into the Afghanistan garbage, Iraq garbage ... Listen, our phone call is being monitored." Watch Zazi being taken into custody by FBI agents »

The probe emerged after a series of raids in Queens last Monday, three days after the intercepted phone calls. A police stop of Najibullah Zazi during a visit to New York had raised investigators' concern that he would discover he was under surveillance, a former counterterrorism official familiar with the investigation told CNN last week.

Najibullah Zazi, who spent three days being questioned by FBI agents last week, and his father were arrested Saturday night in the Denver, Colorado, suburb of Aurora, the Justice Department said. Zazi, 24, and his 53-year-old father are expected to make their first appearance in a Colorado federal court Monday, while Afzali, 37, is scheduled to appear in a New York court the same day. Watch why authorities wanted to talk with Zazi »

All three are originally from Afghanistan. Muhammed Wali Zazi is a naturalized U.S. citizen, while Afzali and Najibullah Zazi are permanent legal residents. If convicted, each faces eight years in prison.

The Zazis and Afzali, a Muslim cleric and funeral director from the New York borough of Queens, are among several people under investigation as part of a plot to detonate bombs in the United States, according to the Justice Department.

However, federal agents have "no specific information regarding the timing, location or target of any planned attack," David Kris, the assistant attorney general for national security, said in a statement announcing the arrests Saturday night.

At the Queens residence where Najibullah Zazi stayed during his visit, FBI agents seized a black scale containing several AA batteries. Zazi's fingerprints were on both, the criminal complaint against him said.

And agents searched his rental car and laptop computer while he was there, turning up what they said were handwritten notes of his explaining how to make, handle and detonate explosives, according to the affidavit.

Both Najibullah Zazi and his attorney denied reports that bomb-making instructions were stored on the suspect's computer. That denial makes up the heart of the charge against him.

Afzali is accused of tipping off the father and son that investigators had approached him for information about them. The criminal complaint against Afzali said he falsely asserted that he never told Zazi that agents were monitoring him on the phone, nor did he ask Zazi about evidence in his rental car.

And Muhammed Wali Zazi told investigators that he had not called, nor had he received a call, from anyone in New York asking him about his son's activities. He also said he did not know anyone named Afzali -- despite recorded phone calls that showed he did, the FBI stated.

An FBI handwriting expert determined the handwriting in the photographed document was similar to Najibullah Zazi's, according to the complaint. But during an interview with investigators Wednesday, he denied seeing the instructions and said that if it was found on his computer, he must have unintentionally downloaded it as part of a religious book. He said he deleted the book within days after he realized that its contents discussed a holy war, investigators said.

Authorities said he gave three days of interviews with the FBI as part of a proffer agreement -- a written understanding that allows a person to provide information about a possible crime without his words being used against him at trial. During those interviews, investigators said Najibullah Zazi admitted he attended courses and received instructions on weapons and explosives at an al Qaeda training facility in the Federally Administered Tribal Areas of Pakistan during a 2008 trip there.

But Arthur Folsom, Zazi's attorney, denied those claims. And a source familiar with the investigation told CNN that no plea deal or proffer was made to Najibullah Zazi during his questioning.

The plot may have been targeting a major transportation center, like a large railroad or subway station, sources close to the investigation told CNN on Thursday. There were plans for an attack, presumably in the New York area, where crowds are large and security screening for nonairport travelers is lax, the sources said.

Two sources familiar with the investigation said the younger Zazi had video of New York's Grand Central Terminal, a massive junction of rail and subway lines, as well as shops and restaurants, which see an average of more than a half million visitors per day.

Speaking at an appearance with Mayor Michael Bloomberg, New York Police Commissioner Ray Kelly said the investigation "is only just beginning." And Bloomberg said the investigation is a reminder "that terrorism hasn't gone away."

"I feel safe walking the streets," he said. "I feel safe having my kids live here. But that's only because we keep our professionals trained and funded and up to strengths necessary to do the best jobs we can to protect us all."

Najibullah Zazi left the United States for Peshawar, in northwestern Pakistan, in August 2008 and returned home the following January, according to affidavits. Peshawar is the capital of the North West Frontier Province, which intelligence analysts say is a haven for Islamic militants who have launched attacks inside Pakistan and targeted U.S. forces in neighboring Afghanistan.

Zazi, a limousine driver for a Denver-area company, told The Denver Post he made trips to Pakistan to visit his wife.

CNN's Kathleen Johnston and Alona Rivord contributed to this report.

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