Black Horizon

CNN) -- A terror plot that came to light this week following raids in New York may have been targeting a major transportation center, sources close to the investigation told CNN on Thursday.

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

There was planning and preparation for an attack, presumably in the New York area, where there would be a large number of people and where security screening is lax such as a large railroad or subway station, essentially where there is no airport-style screening, the sources said.

Authorities are taking the plot seriously, because, the sources said, they think it involves "real-deal terrorists" operating and planning an attack in America.

Because of that fear, an unprecedented level of resources is being devoted to the investigation, the sources said. That includes the placement of a hostage rescue team in New York for possible raids and the deployment of additional resources to the Denver area in Colorado, where another phase of the probe is taking place.

The federal terrorism probe emerged Monday with a series of raids in the New York borough of Queens.

A former counterterrorism official who has been briefed on the investigation said bomb instructions were found, but could not say where.

The former official said backpacks, computers and maps were found during the searches in New York, and field tests turned up positive for explosives. But initial tests often yield false positives, and the former official was unaware whether more definitive tests had been concluded.

The backpacks support a theory that an attack similar to the 2004 train bombings in Madrid, Spain, was being planned.

The Madrid bombings -- coordinated attacks on four morning-rush commuter trains -- killed 191 people and wounded more than 1,800.

A 24-year-old Colorado resident is at the center of the probe and met Thursday with FBI agents in Denver for a second day as his lawyer disputed a report that bomb-making plans were found on the man's computer. Watch why Colorado man is being questioned by authorities »

Federal agents searched Najibullah Zazi's apartment and another home in the same Denver suburb on Wednesday in connection with the terrorism 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, Arthur Folsom, dismissed that allegation.

"There's no diagram of a bomb. There's no information like that," Folsom told reporters as he walked his client to his second meeting with federal agents. If something like that had turned up on Zazi's computer, he asked, "Do you really think the FBI would have allowed us to walk out of here last night?"

Zazi, an Afghan national, gave writing, fingerprint and DNA samples to FBI agents Wednesday during a "very friendly, very cordial" interview, Folsom said. He said Zazi has no ties to terrorism, and he thinks his client drew investigators' attention "because he stayed at a house owned by an old friend of his who was under observation from the FBI."

According to law enforcement sources with knowledge of the investigation, 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 New York police informant, with authorized FBI wiretaps used to further 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, raising concerns that he would figure out he was under surveillance, the former official told CNN.

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