CAMP BUCCA, Iraq — A riot three weeks ago that resulted in guards shooting to death four prisoners at the biggest U.S.-run detention facility in Iraq has exposed an increasingly hard-core prison population that is confronting U.S. forces with a growing risk of violence, according to military officers.

U.S. troops who dealt with the clash tell of a chaotic and violent situation that surprised them. They also say the nonlethal weapons available to them at the time for crowd control proved largely ineffectual.

"What happened here on January 31st has changed the dynamics" of managing such situations, said Maj. Gen. William Brandenburg, who oversees U.S. military detention operations in Iraq. "It showed that the prisoners could hurl rocks farther than we could fire nonlethal weapons. It also showed that we have to do a better job of understanding who we have in detention."

Frightened guards, some having arrived in Iraq only a month before, tried vainly to quell the riot, spraying pepper gas and shooting rubberized pellets into throngs of prisoners, according to accounts by troops here.

The clashes spread through five compounds at the sprawling detention facility in the southern Iraqi desert near the Kuwaiti border. Prisoners pelted guards with large stones and makeshift weapons, heaving debris over 15-foot-high metal fences and up at 30-foot-tall guard towers that ring the compounds.

Only after two Army guards in separate towers opened fire with M-16 rifles, killing the inmates and wounding six others, did the violence subside. U.S. officers say the guards acted on their own, with no order to fire.

The episode remains under criminal investigation by the military, although rules here allow for use of deadly force if soldiers feel endangered.

Camp Bucca, which houses 5,150 prisoners, about 1,000 below capacity, is divided into eight compounds. Tents in which prisoners were housed have given way, in many cases, to climate-controlled huts built of wood with corrugated metal roofs. Small fields are available for soccer or volleyball games. Hot meals of rice, soup and stew are served.

There had been trouble at Camp Bucca before. In mid-October, fighting broke out between Sunni and Shiite prisoners in a dispute over observance of Ramadan, the Muslim fasting month. The Shiites were later placed in a separate compound.

In early December, a protest erupted after two prisoners were sentenced to isolation for an escape attempt. Shouting prisoners, armed with sticks from collapsed tents and shielded by mattresses, threatened to assault. Guards avoided firing and, instead, dispersed the inmates using extra troops, guard dogs and firetrucks.

The uprising of Jan. 31 began when U.S. soldiers entered compound No. 5 to search for contraband. A Muslim cleric complained that the soldiers damaged several Qurans. Soon, masses of prisoners formed and pressed against the compound's front fence, chanting and shouting.

"The initial worry was that they would push the fence over and escape," said Air Force Tech. Sgt. Keith Gray, who rushed to the scene with an emergency-response force of 15 troops. "We started spraying gas, which pushed back the first row or two. But then they started throwing things."

Using makeshift slingshots, the inmates hurled rocks and chunks of concrete torn from the floors of their huts. They tossed sticks and plastic water bottles filled with sand. They lit plastic bags filled with flammable hand sanitizer.

Prisoners in four other compounds quickly joined in.

Senior Airman Tony Miles, who was manning a tower at compound No. 1, found himself trapped by flying debris. "It was chaotic," he recalled. "Stuff was coming from everywhere."

In another tower, Airman 1st Class Eric Coggswell repeatedly shouted in Arabic for the prisoners to stop. "But they weren't listening," he recounted. "I fired eight shotgun rounds of nonlethal rubber bullets and small rubber pellets. But a lot of the prisoners were using sleeping bags as shields."

Other guards said the inmates appeared to know the limited ranges of the nonlethal shotgun blasts and gas sprays and would withdraw out of range, then rush again toward the perimeters. Their rocks shattered the windows of some tower huts.

"It was like an upside-down water fountain, with projectiles spewing into the towers," said Army Capt. Jerry Baird, who supervises the internment facility.

Army Pvt. 1st Class Christopher Cole described the prisoners as moving in waves around the perimeter of his compound, aiming at one tower and then another.

"When they were in front of my tower, there were so many rocks being thrown that I couldn't do anything except crouch in back of the tower hut until they passed," he said. "But then my nonlethal shots wouldn't reach them."

The rioting lasted about an hour, ending soon after word spread that several inmates had been killed.

U.S. commanders suspect the uprising was planned, although the purpose remains unclear. Some here suspect it was meant to protest the Iraqi elections, which had been held the previous day. Others say they believe it was designed to test the guards' responses.

Under U.S. military rules, none of the inmates could be interviewed for this article.

Senior officers here defended the decision by the two military policemen — both sergeants — to fire deadly shots.

"That is a judgment call for the MPs to make," said Lt. Col. Tim Houser, commander of the 105th Military Police Battalion, which is in charge of Camp Bucca. "They felt there was potential for loss of life or grievous bodily injury."

Guards now have new, nonlethal guns that shoot longer-range, plastic projectiles. In addition, the camp has received a 1,000-gallon firetruck whose spray can be used against rioting inmates.

Brandenburg has ordered more cameras installed at all compounds to intensify surveillance, and he has instructed officers to do more to gather intelligence on the prison population.

"You can't do enough to figure out who the bad guys are," he told the senior staff at a meeting last week. "We're detaining a harder-core crowd, and so the approach has to be more prisonlike. We've got to get very good at this to get ahead of it."

A maximum-security facility with segregated metal cells is being built at the camp. Brandenburg said it should help set troublemakers apart.