As New York City transportation officials moved to ratchet up security at major rail hubs, law enforcement sources said Tuesday that police were conducting more visits to truck rental companies and chemical suppliers as part of the major terrorism probe that led to the arrest of three men over the weekend.

Reports also spread that police had raided homes in Queens in search of more than a dozen associates of Najibullah Zazi, 24, the Afghan immigrant who has emerged as the central figure in the investigation into a possible plot to set off explosives in the city.

Photo credit: Ari Mintz | MTA police officers, Mike Rees and Ray Weingart, keep watch on the Long Island Rail Road concourse in Penn Station on Tuesday. (Sept. 22, 2009)

Zazi admitted that he was trained at an al-Qaida camp in Pakistan and had bomb-making instructions on his computer, according to court papers filed in federal court in Denver.

Police and FBI officials said they were unaware of any such raids or the execution of any additional search warrants in the investigation, which has stretched from Colorado to New York and Afghanistan.

Still, transportation officials were taking heightened action in light of the investigation.

"We have increased police presence at key locations in light of the ongoing investigation," said a spokesman for the Metropolitan Transportation Authority.

Speaking to reporters separately later in the day, Mayor Michael Bloomberg and Police Commissioner Ray Kelly downplayed the issue of heightened security in the transit infrastructure, which some security experts believe could be targeted for a Madrid-style attack, which took nearly 200 lives in 2004.

"That is not what's happening in New York because we have been at a heightened level [of security] for a long time," Bloomberg said. "And I think everything that they recommended we already do."

Speaking at a charity event in SoHo, Kelly said the NYPD has been conducting security to fit the needs of the city, particularly at a time when world leaders are in town for the United Nations. He declined to discuss the investigation in any detail.

Later Tuesday, the MTA amplified its position and said it was using more of its own police officers on the commuter rail system and at major rail hubs as the ongoing probe continued.

In the week since the terror investigation broke into the open with raids in Flushing, Zazi, his father, Mohammed, 53, and Queens Imam Ahmad Wais Afzali, 37, were arrested on charges they lied to the FBI, but they haven't been charged with terrorism. The three were being held without bail - the Zazis in Denver and Afzali in Brooklyn. Officials also have noted that there were no specific threats or plans to target any locations in New York.

A private intelligence expert in Washington, D.C., told Newsday that there is a view among some law enforcement officials that the Joint Terrorism Task Force may have acted too soon in the New York case.

The analyst, who did not want to be named, said one indication the alleged plot was embryonic and ill-formed was that the alleged associates of Najibullah Zazi had not developed any plans to flee if something happened.

"There were no financial movements," said the expert.

However, Zazi had racked up $51,000 in credit-card charges in less than a year, from mid-2008 to March, at a time when he was earning $800 a month, according to bankruptcy filings.

Records aren't clear about what he was doing with the money.

With Pervaiz Shallwani

www.newsday.com/news/new-york/nyc-terror-probe-cops-visit-truck-rentals-chemical-suppliers-1.1466563