A U.S.-Canada border checkpoint is being investigated in connection with Thursday's bombing of a military recruiting centre in New York's Times Square.

Several U.S. media reports say four men who tried to cross the border into Canada about a month ago were pulled over by Canadian officials for an inspection. It's not clear where it happened, but CBS and NBC report that two of the men fled, while ABC says one of the men ran away.

In their inspection, border officers reportedly found a backpack with photos of various New York City locations including Times Square and the military recruitment centre.

Law enforcement officials described the men as anarchist types, the reports said.

ABC News quotes sources who say an investigation at the time did not turn up any evidence of criminal activity, but U.S. and Canadian authorities now are revisiting the incident.

Canadian authorities declined to comment on the matter to ABC.

No link to letters, photos

Also Friday, U.S. officials said there is no link between the bombing and letters sent to Congress saying "We did it."

The lengthy anti-war letters — which arrived with photos of a man standing in front of the recruiting office before it was damaged — contained no threats and were sent to as many as 100 members of Congress, officials said.

Laura Eimiller, an FBI spokeswoman in Los Angeles, said an individual was questioned there about the letters to Congress and "there is no evidence linking the letters, which contained no threat, to the bombing."

A law enforcement official in Washington, who spoke on condition of anonymity because the investigation of the bombing is continuing, called the timing of the letters an "incredibly unbelievable coincidence" and said no charges were expected in connection with them.

He said the "We did it" referred to the Democrats winning control of Congress in 2006.

Democratic lawmakers on Capitol Hill were startled to receive the letters in their office mail Thursday afternoon, just hours after the early morning New York bombing, and turned them over to the Capitol Police.