Frank McKenna (file photo)

A Montana senator says he "misspoke" when he erroneously claimed that the people who carried out the Sept. 11 attacks entered the United States from Canada.

Sen. Conrad Burns sent a letter on Tuesday to Canada's ambassador to the United States, apologizing for his comments at a news conference a day earlier.

"I misspoke on this matter," Burns, a Republican, writes in the letter to Ambassador Frank McKenna.

"And I ignored my notes indicating my sincere belief that securing the border is vitally important to not only the United States but Canada as well."

But he insisted his mistake shouldn't deflect focus from his main concern, saying security problems "clearly exist on the border."

Burns cites the case of Gazi Ibrahim Abu Mezer, who had planned to bomb a New York subway and was arrested in 1997 on his third illegal entry into the United States from Canada.

He also noted that Ahmed Ressam, who plotted to bomb the Los Angeles International Airport during the Millennium celebrations, was arrested at the B.C.-Washington border on his way south in 1999.

"These incidents are disturbing and should not be ignored," Burns wrote.

"We still have much work ahead of us. ... The threat of terrorists crossing from the United States to Canada is just as real and those who wish to do harm should not be able to cross a porous border to carry out an attack in either country."

McKenna demanded a retraction from Burns on Monday, after he told reporters at a news conference on border security: "We've got to remember that the people who first hit us in 9/11 entered this country through Canada."

McKenna said he felt he had to speak up because the remarks were "patently false" and could seriously mislead Americans over the events that preceded the 2001 attacks.

Even though investigators have determined that not one of the attackers entered the United States from Canada or Mexico, the mistaken belief has persisted among some Americans.

McKenna has tried aggressively to challenge the myth, which raises hackles in Ottawa.

In the spring, another prominent Republican – former House speaker Newt Gingrich – apologized to Canada after telling a television network that "far more of the 9/11 terrorists came across from Canada than from Mexico."

The U.S. House of Representatives passed a border security bill on Dec. 16, which includes an initiative to study building a security wall along the Canada-U.S. border.

Canadian officials said the issue had never been raised by U.S. officials during regular joint meetings on border security.

They said Canada had no interest in a wall.