Attorney General John Ashcroft announced Tuesday his department continues to detain 603 people on immigration violations or other federal charges in the terrorism investigation, and some are believed to be members of the Al Qaeda terror network.

Ashcroft said those detaiend were pulled in as part of a "deliberate campaign" aimed at disrupting terrorist activities.

"The Department of Justice is waging a deliberate campaign of arrests and detentions to protect American lives.  We are removing suspected terrorists who violate the law from our streets to prevent further attacks.  I believe we have Al Qaeda membership in custody and will use every tool to keep suspected terrorists locked up," Ashcroft told reporters in a Tuesday news conference.

The attorney general's announcement was the first accounting of people in federal custody in several weeks. Previously, the government had said only that it detained or arrested more than 1,100 people without identifying them or saying which remained in custody.

Ashcroft said the terrorism investigation had charged 104 people with federal crimes, and that 55 of those remained in custody. In addition, he said, 548 people remain in custody on immigration charges, some of whom are also charged with federal crimes.

Ashcroft defended the secrecy of the investigation, including the fact that most of the names of the detainees have not been public, saying he wanted to protect the privacy of people who may be innocent and prevent bin Laden from gaining valuable information.

"I am not interested in providing, when we are at war, a list to Usama bin Laden and the Al Qaeda network of the people we have detained that would make any easier their effort to kill Americans," Ashcroft said.

The announcement comes amid concerns in Congress and among civil rights groups about the secrecy surrounding those who have been detained, most of whom are believed to be of Middle Eastern descent.

Those concerns were not reassured after law enforcement officials in Michigan said they are trying a new approach to investigating possible terror links.

Officials in Detroit, home of the largest concentration of Arab Americans, has contacted about 700 of the 5,000 non-citizens sought for questioning nationwide by the Justice Department.

Instead of sending investigators door to door, the FBI's anti-terrorism task force in the Detroit area is sending out a letter to hundreds of Middle Eastern men who have come to the United States on temporary visas in the last two years.  The letter asks them to make appointments for interviews to talk about any information they may have surrounding the Sept. 11 terror attacks.

Ashcroft defended the measure as a means to focus on people with visas issued in countries from which terrorists came to the United States.

"We're being as kind and fair and gentle as we can in terms of inviting people to participate in helping us. And we have not identified people based on their ethnic origin; we have identified individuals who are not citizens, but based on the country which issued their passports," he said.

Ashcroft said the new approach should not be seen as an affront since everyone living in the United States should want to cooperate in fighting a terror war.

 The Associated Press contributed to this report.