WASHINGTON - Illegal immigrants who volunteer to leave the country through an experimental government program may have to wear tracking devices or check in at offices until they go, Immigration and Customs Enforcement said Thursday.

Between Tuesday and Aug. 22, people who have ignored deportation orders and have not committed crimes can show up at ICE offices and arrange to leave the country. The offer will only be available in five cities: Santa Ana, Calif.; San Diego; Phoenix; Charlotte, N.C. and Chicago.

About 475,000 people have deportation orders and have not committed crimes, said ICE spokesman Richard Rocha. ICE did not know how many such people are in the five cities.

Those who volunteer will have up to 90 days to take care of personal affairs before leaving.

During that time, volunteers could be required to wear an electronic monitoring device on an ankle confining them to certain areas.

In some cases, those who want to appeal a deportation order could be detained while they fight the order, depending on the circumstances of their cases, Rocha said.

Volunteers might get help with flights or bus rides home if they can't afford the travel. But laws prohibiting people who have been in the country illegally from returning legally for 5 to 20 years would still apply, Rocha said.

Rocha said that by volunteering to leave, immigrants will be able to prove a departure date when they apply to enter the U.S. legally years later.

Congress had considered so-called "report-to-deport" proposals requiring illegal immigrants to report to federal offices, but they failed to become law.

The self-deport idea has left some advocacy groups puzzled.

"The program is a head scratcher, to think people are going to come forward and walk away from their life here. It shows how desperate and delusional this administration has gotten in dealing with illegal immigration," said Angela Kelly, director of the Immigration Policy Center of the American Immigration Law Foundation.

Frank Sharry, executive director for America's Voice, said the plan appears to be a political move to convince Republicans the administration is now serious about immigration enforcement.

"I'm going to go out on a limb here and say there won't be lines around the block for this program," Sharry said.

Also Thursday, the Congressional Hispanic Caucus voted to ask for investigations by the Justice, Homeland Security and Labor departments of the May immigration Agriproccessors meatpacking plant raid in Postville, Iowa where nearly 400 people were arrested.

Rep. Joe Baca, D-Calif., the caucus chairman, and two other Hispanic caucus members visited Postville over the weekend and heard emotional testimonials from people affected by the raid.

Members voted unanimously to ask for an investigation of the raid. They also want more congressional hearings.

"Who gave them the authority to do something like this?" Baca said of the Immigration and Customs Enforcement officials who conducted the raid, the largest of its kind in U.S. history. "This reminds us of slavery how these people were treated and how they were coerced" into pleading guilty to criminal charges.

Associated Press writer Erica Werner contributed to this report