FBI agents are expanding their search for a group of foreign-born men they believe may have crossed into the United States illegally from Canada in a false ID case that has heightened terrorism fears around the New Year's holiday, government officials said Tuesday night.

The officials, some of whom work in law enforcement, told The Associated Press the government has identified several more men it fears may have used fake passports to get into the country around or after Christmas Eve.

Authorities identified five such men on Sunday and asked the public's help in identifying them.

The FBI and Homeland Security officials are considering making the names and photos of about a half dozen more men public as early as Wednesday, the officials said, speaking only on condition of anonymity.

Officials cautioned they have no specific evidence the men are involved in a terrorist plot, but said the men may have connections to a fake ID and smuggling ring that involves some people with terrorist connections. Much of the search effort has focused on New York City.

But the broader investigation into fake IDs has spanned several countries, including Canada, Pakistan and Britain, the officials said.

Because intelligence indicated several men were trying to use fake passports to get into the United States before New Year's Day -- a holiday of symbolic significance to Islamic terrorists -- U.S. officials were making aggressive efforts to track them down and make sure they posed no terrorist threats, the officials said. Several of the men have names or passports from countries with large terrorist presences, further heightening concern, the officials said.

The additional names expected to be made public Wednesday have come from the same Canadian intelligence sources that identified the five men whose identities were released over the weekend, the officials said.

The expanding search came as federal authorities received unsubstantiated information that terrorists could launch a maritime attack on New York City, a threat taken seriously because of the large crowds that gather in Times Square on New Year's Eve.

Gordon Johndroe, a spokesman for the new agency, said the government was reacting similarly to July 4, when security was increased mainly because of the crowds rather than information from specific threats.

Johndroe said the information on the new threat was "unsubstantiated, uncorroborated and of suspect credibility."

There are no plans to raise the nation's threat level above yellow, the middle of a five-tiered color-coded rating system, Johndroe said.

Meanwhile, President Bush said Tuesday he personally ordered the FBI to begin a nationwide hunt for the five men believed to have entered the country from Canada on Christmas Eve.

Speaking in Crawford, Texas, where he's vacationing at his ranch, Bush said U.S. authorities need to know what the men are doing in the United States.

"We don't have any idea of what their intentions might be, but we are mindful that there are still some out there who would try to harm America and harm Americans and so therefore we take every threat seriously, every piece of evidence seriously," Bush said.

The FBI on Sunday released photos, names and birth dates of the five and sought the public's help tracking them down for questioning. However, bureau officials acknowledge they are not certain that the identities and ages are correct.

The five, described by the FBI as men of Middle Eastern origin, may have fake passports since the information that led to the FBI bulletin came from an investigation into an illegal passport scheme.

The names released by the FBI are Abid Noraiz Ali, Iftikhar Khozmai Ali, Mustafa Khan Owasi, Adil Pervez and Akbar Jamal. Each has a purported birth date between 1969 and 1983.

All remained at large Tuesday.

Sen. Hillary Rodham Clinton, D-N.Y., told a news conference Tuesday it's believed the five crossed the border into New York state with false documents. She said the Department of Homeland Security needs to establish an office dedicated to improving security along the nation's northern border.

The Toronto Sun, quoting police sources in Canada, said the men arrived at Toronto's Pearson International Airport about two weeks ago, claiming refugee status. They stayed in the Toronto area for several days before being smuggled into the United States by car.