More than 700 people were arrested on immigration violations and thousands more subjected to FBI (search) interviews in an intense government effort to avert a terrorist attack aimed at disrupting the election.

As with past unrealized Al Qaeda (search) threats, law enforcement officials said Thursday they don't know for sure whether any of those arrests or interviews foiled an attack.

"It's very hard to prove a negative," Michael Garcia, chief of Immigration and Customs Enforcement (search), said in an interview Thursday. "We did cases and operations for people we thought posed national security concerns. We didn't arrest anyone who had a bomb."

For example, ICE agents arrested a 23-year-old Pakistani man in late October who had illegally entered the United States through Mexico in 2000 and was working as a fuel tanker truck driver with access to a major U.S. seaport. The man, who was not further identified, is charged with making false statements about how he entered the country and remains under investigation for any links to terrorism.

He was one of the 237 people arrested in October alone on immigration violations, for a total of over 700 since the enforcement effort began last year, Garcia said. "It was a broad approach that led us to have a very disruptive effect, we believe," he said.

Although the election season passed without an attack, officials say Al Qaeda remains a dangerous foe intent on striking the United States again. The day after the election, Attorney General John Ashcroft told his senior staff to not let their guard down.

The Jan. 20 presidential inauguration heads the list of upcoming high-profile events that officials say could draw terrorist interest. Others include the Feb. 6 Super Bowl in Jacksonville, Fla., and the December holiday travel season, which last year saw several threats against trans-Atlantic flights.

There still is concern the Usama bin Laden videotape aired last week could be a signal for an attack. And despite asking for help from the public, the FBI still has not identified a man calling himself "Azzam the American," whose lengthy videotape aired last month promised attacks that will make U.S. streets "run red with blood."

The FBI interviewed about 10,000 Muslims and Arab-Americans in the months prior to Election Day in an effort to gain intelligence about people who might pose a threat and to build bridges to those communities.

Many of those interviews led officials to individuals in the United States who might be linked to terrorism but had previously escaped government detection, said a senior Justice Department official speaking on condition of anonymity because of national security concerns. The official did not provide any details.

Still, there were reports of heavy-handed tactics in some places. The Council on American-Islamic Relations provided several examples, including a young Pakistani man who was held for five hours in Las Vegas after books on the Muslim holiday of Ramadan and Arabic grammar were found in his possession.

"This was viewed as an extension of the ongoing policies that have been targeting Muslim and Arab-American communities," said CAIR spokesman Ibrahim Hooper. "These communities view themselves as law-abiding and contributing to society in a very positive way."