LEWISBURG, PA. -- Edel Alvarez came to America for the good life.

Imprisoned in Cuba as "a risk to the state," he left the island nation -- and his grandparents and his 26 brothers, sisters, stepbrothers and stepsisters -- in May 1980 to join the flood of refugees departing the port of Mariel.

But, although he found work as an electrician in Fort Walton Beach, Fla., the good life did not come fast enough. Alvarez decided to hasten the process by stealing.

He twice was caught shoplifting, was sentenced to a year in a county jail and served his time without incident.

But now, 5 1/2 years later, Alvarez, 34, remains in prison. He is allowed out of his cell at the maximum-security federal penitentiary here for about eight hours each week for showers, recreation and exercise. The rest of the time he remains locked alone in Cell 304.

He is one of 125 Cuban "detainees" at Lewisburg, men who have completed their criminal sentences but are being held on instructions of the U.S. Immigration and Naturalization Service (INS) as potential threats to American society. Half of the detainees are in "lockdown," confined to their cells an average of 23 hours a day, more closely restricted than most of the general prisoners who are serving long criminal sentences.

"We can do nothing -- no work, no school, no nothing," Alvarez said. "We can do nothing to get ready for the future."

Nationwide, more than 2,600 other Cubans also are legal phantoms: prisoners who have served their time but who remain in prison, unable to be deported, unable to go free, and often unable to share the prison liberties that regular prisoners have.

They are the remnants of the 1980 Mariel boatlift and the legacy of the 1987 prison riots in Atlanta and in Oakdale, La. They are in a legal limbo that may last as long as they live.

"It's clearly legal," said Elizabeth Marsh, a former New Jersey college professor who now directs the Lewisburg Prison Project, which works for improved conditions for the detainees. Federal courts have ruled that the INS may hold the Cubans indefinitely. But, Marsh said, "it's a violation of civil rights under the cover of law."

"They are just being stored," she said. "Not knowing what is going to happen next is the hard part for them. Some have tried to kill themselves. A lot do things to make a life for themselves. . . . They try to learn English from the radio; they run in place. A lot get religion. At night, the lights are on, and sometimes you can hear them crying."

Since they are not U.S. citizens, the Cubans do not have the constitutional rights of American citizens. Since they did not enter the country legally, they do not have the protections afforded legal aliens. Since they came from a country that will not take them back, they cannot be deported. The federal government is spending $55 million a year to keep them behind bars, and no one knows when they will get out.

An additional 3,500 "Marielitos" who are serving criminal sentences in local and state jails will eventually be in the same situation.

"I don't know where we're going to end up," said Patrick W. Keohane, warden of the Lewisburg prison. "It's a very difficult situation for everyone -- for the managers, for the {regular} inmates, for the detainees."

Victor Bueno-Bell, a Cuban who has been a detainee since he completed an 18-month sentence for marijuana possession in 1986, spends his days in a 7-by-14-foot cell surrounded by books and pinup photos and pictures of his wife and stepdaughters.

"I'm tired, I'm so tired of this," said Bueno. "I've got three years in lockdown, three years in this . . . cell, 24 hours a day. Always handcuffed -- go to the shower, go to the TV room, always in handcuffs."

Most of the non-Cuban prisoners at Lewisburg have limited freedom to move about within prison walls and most are not regularly handcuffed. But prison officials are wary of the Cubans, many of whom were at the Atlanta prison during the 1987 riot that destroyed it. And about three months ago, 38 Cubans on one floor of the Lewisburg prison erupted in anger when a guard declined to get a radio part for one prisoner. They damaged some of their cells and "created a pretty good disturbance" for about an hour, Keohane said.

"Those that are disruptive -- I don't think they have anything coming," Keohane said. "Cooperation is the way out of there {lockdown}. If they have a clear-conduct record, they can move into the general {prison} population."