NEW YORK (CNN) -- Najibullah Zazi, a Colorado resident and Afghan national questioned in an alleged terrorist plot in the United States, declined to attend a fourth day of interviews with the FBI, his attorney's publicist told CNN on Saturday.

A lawyer for Najibullah Zazi disputes claims that bomb-making plans were found on his client's computer.

No other details were available from attorney Arthur Folsom's office, though a statement was expected later Saturday.

The federal terror probe that snagged Zazi began in Denver, Colorado, and expanded to New York, where authorities Saturday confirmed that they searched a U-Haul location where a group of Afghan men unsuccessfully tried to rent a large truck this month.

Zazi has admitted to having ties to al Qaeda, an administration official familiar with the matter told CNN on Friday, but has not been arrested or charged.

Either a plea deal or charges are possible for Zazi, the official said.

The alleged terrorist plot, which came to light this week after raids in New York, may have been targeting a major transportation center, like a large railroad or subway station, sources close to the investigation told CNN on Thursday. Watch the latest on investigation into alleged terror attack »

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.

The investigation in New York has led authorities to a U-Haul location in the New York City borough of Queens, where the men allegedly tried to rent the truck.

A former counterterrorism official familiar with the investigation said that the group of Afghans tried to rent the truck September 9, but were unsuccessful after three different credit cards they attempted to use were declined. The men, who held driver's licenses from Florida and Ohio, also were unable to use cash for the rental, the official said.

The FBI searched the U-Haul location Thursday, the official said.

U-Haul spokeswoman Joanne Fried confirmed the search and said the company is cooperating with the investigation. U-Haul staff identified one of the Afghans to the FBI as Naiz Khan, the former counterterrorism official said. Khan, whose apartment was raided by authorities earlier this week, had given Zazi a place to stay last weekend. Watch more about the U-Haul location and the men being investigated »

In a CNN interview Saturday, Khan denied that he was at the U-Haul store and said he had no knowledge of a plot.

Khan said he doesn't know about any ties Zazi had to terrorism, saying that he only knew Zazi from a mosque in Queens. Khan, 26, said he runs a coffee cart for a living six days a week.

Khan was prepared to leave the country Saturday to visit his wife in Pakistan for four months, he told CNN, but decided to postpone the trip until the issue is resolved.

"While DHS and FBI have no information regarding the timing, location or target of any planned attack, we believe it is prudent to remind transit authorities to remain vigilant," the Department of Homeland Security said Friday in a written statement.

Two sources familiar with the investigation said that 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.

A former counterterrorism official said backpacks, computers and maps were found during searches in Queens, and field tests turned up positive for explosives. But such tests often yield false positives, and the former official was unaware whether more definitive test results had been obtained.

On Wednesday, federal agents searched Zazi's apartment in Denver and another home in a Denver suburb in connection with the investigation.

A law enforcement official told CNN that diagrams showing how to make bombs were found on the computer that Zazi had with him when he was stopped in New York during a recent visit, but his lawyer, Folsom, denied that was true.

"There's no diagram of a bomb; there's no information like that," Folsom told reporters Thursday as he accompanied his client to his second meeting with federal agents. Had that been the case, he asked, "Do you really think the FBI would have allowed us to walk out of here last night?"

Zazi has no ties to terrorism, Folsom said.

He suggested that Zazi may have drawn investigators' attention "because he stayed at a house owned by an old friend of his who was under observation from the FBI."

Folsom said Zazi stayed in an apartment that was raided after he had driven from Denver to New York on business. Sources close to the investigation told CNN that the Queens raids were spurred by a confluence of events in the city, including the upcoming U.N. General Assembly session and President Obama's Wall Street speech on Monday.

Law enforcement sources with knowledge of the investigation said the Colorado searches were part of a probe that began with Zazi and led to New York.

Zazi has driven limousines for First ABC Transportation in suburban Aurora, near Denver International Airport, for about six months, according to a worker who answered the phone at the company.

The man, who identified himself only as "Joe," said he was startled to hear Zazi was under investigation. He said Zazi was a hard-working man who was single-handedly supporting his family.

"He is a young, nerdy, kind of good kid -- nothing to do with religious or anything," the man said. "He is a kid."

Joe said co-workers called Zazi "the bearded one" in a lighthearted way. When he heard that Zazi might be associated with a bomb plot, he said, "I was literally laughing."

"I agree with his lawyer he has nothing to do with that kind of stuff. His character is much better than that," Joe said.

The case began with a tip from a New York police informant and led to FBI wiretaps to develop the case, the former counterterrorism official said.

Agents launched the raids after police stopped Zazi on the George Washington Bridge during a recent visit to New York City, raising concerns that he would figure out he was under surveillance, the former official told CNN.

FBI Director Robert Mueller told a Senate committee Wednesday that he did not think the investigation had revealed any "imminent danger."

Resources devoted to the investigation include the placement of a hostage rescue team in New York for possible raids and the deployment of resources to the Denver area in Colorado, where another phase of the probe is taking place, the sources said.

CNN's Deb Feyerick, Kiran Khalid and Jeanne Meserve contributed to this report.

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