A new Congressional report finds the Justice Department has been unable to locate nearly half of the registered immigrants in the United States who are wanted for questioning in connection with the investigation into last year's terrorist attacks. The Immigration and Naturalization Service is again being blamed for failing to keep track of people entering the United States.

Federal investigators wanted to question more than 4,100 immigrants in the days after last year's attacks. Even though all of them entered the country legally, authorities could not find nearly half of them. Why?

"One of the reasons is that, despite the rules that they're supposed to send in to INS a notification when they change addresses, most people don't do that and INS therefore does not know where they are when they've moved," said Norm Rabkin, a managing director at the General Accounting Office, the investigative arm of Congress that issued the report.

The report points out that once immigrants enter the United States, they have no incentive to tell the INS when they move, especially if their motive is to break the law and overstay their visas, as many do.

"The practical issue here is that if an alien does not want the INS to know where they are, they are not going to fill out the form and tell INS and INS is really not going to know if they have addresses for everybody," he said. "It's almost impossible to do."

The INS has now put in place a new system to better keep track of the estimated half a billion people who legally enter the United States during a typical year.

"If somebody willfully does not give us a change of address, then there are penalties involved in that and of course those are the hardest people to find because generally speaking, if they willfully will not give you the change of address, it's because they are hiding something," said INS spokeswoman Danielle Sheahan.

This is not the first time the INS has come under criticism in the wake of last year's attacks. In what President Bush called an inexcusable incident earlier this year, the agency sent notices to an aviation school in Florida letting it know that student visas had been approved for two of the hijackers involved in the September 11, 2001, attacks - six months after the event.

After years of criticism for being slow to handle immigration cases, and a failure to keep track of foreigners in the country, the INS will soon be dissolved and its functions will be transferred to the newly created Department of Homeland Security.