Thousands of federal prisoners remained locked in their cells or dormitories yesterday as the federal Bureau of Prisons assessed and repaired damage from four riots at widely separated institutions last week.

No one was killed, but several inmates and prison staff members suffered minor injuries during the riots. Fires were set in two institutions, destroying a small recreational building in Talladega, Ala., and damaging all five housing areas of the prison in Memphis. Windows, furniture and equipment were damaged at Allenwood prison in White Deer, Pa., and a prison in Greenville, Ill.

The "lockdown" covers 70 of the 84 penal institutions run by the bureau, which is part of the Justice Department. Inmates are confined to their living quarters and are being fed there rather than in open dining areas. Recreation, work and other activities have been canceled. The order does not apply to the bureau's minimum-security institutions.

Officials yesterday refused to say whether they believe the riots were connected to one another, or if a political message was intended.

Several news reports said some of the rioters were upset at Congress's unwillingness to reduce the penalties for crimes involving crack cocaine. Those penalties generally are longer than for some other drug offenses.

"It will probably be a few days before we know what caused them, and whether or not the incidents were related," said Bill Bechtold, a spokesman for the Bureau of Prisons. "We also won't know for a while what issue, or issues, caused them."

There are 90,304 people in federal prisons. An estimate of how many were affected by the lockdown was not available yesterday.

The first riot began about 6:15 p.m. Thursday at the medium-security prison in Talladega. Two inmates quarreled on the recreation yard, and guards were unable to stop the dispute, said Corliss Moragne, a prison spokeswoman. The disturbance spread quickly through the yard, and eventually out of it.

A craft workshop in the recreation area was set on fire, burning to the ground. Fires were ignited in at least three other areas over the next hour, Moragne said. Though several residential areas were damaged by smoke and water, all are habitable, and the prison's 1,150 inmates are in their normal living quarters, she said.

The second disturbance began at 7:30 a.m. Friday at the medium-security prison in Allenwood. The Associated Press reported that about 150 inmates rioted in the dining hall, breaking windows and pulling fire alarms. One prison worker was injured when hot liquid was thrown on her.

The third began at 12:45 p.m. Friday when a group of inmates in an electric cable factory at the medium-security prison in Memphis stopped working. They broke windows, vandalized equipment and then moved into the recreation yard, where several hundred inmates were spending time after lunch.

Mattresses, newspapers and other items were set on fire in three housing units soon after the riot moved to the yard, said Francine Branch, a spokeswoman at the prison. Subsequently, blazes were started in the remaining two housing areas. All were extinguished by about 9 p.m. Three staff members had minor injuries, and four inmates were treated for smoke inhalation.

The housing areas were too damaged by fire, smoke and water to be immediately reoccupied, Branch said. The inmates were being held yesterday in the prison gymnasium and in a cellblock for disciplinary cases. They could return to their usual rooms in the next few days.

Bureau of Prisons officials in Washington declared the lockdown about 4 p.m. Friday. The fourth riot -- at the medium-security prison in Greenville -- began when guards implemented that order.

Details of that disturbance were sketchy yesterday, but a news release from the prison said that damage "was limited to the housing units." News service reports said that several employees erected a barricade during the riot and were rescued by police. Ten members of the prison staff "received injuries, most of which were cuts and bruises," according to the news release. CAPTION: Federal and local officers guard entrance to U.S. prison at Talladega, Ala., after rioting there injured several people.