The first grenade explosion sounded like a large cherry bomb going off at a Fourth of July parade, according to Ron Abney, an American whose leg caught a pellet of metal shrapnel as he watched a noisy demonstration in front of the National Assembly building on Easter Sunday, March 30.

Then came three more grenade explosions in rapid succession, which blew the arms or legs off dozens of other onlookers and led to at least 20 deaths and 150 injuries, transforming a grassy field into a bloody tableau of panic and mayhem. But what happened next, according to witnesses, was as shocking as the blasts.

A cordon of soldiers surrounding the protest opened to allow the passage of several men suspected of throwing the grenades, but closed to block their pursuit by protesters. Other soldiers blocked ambulances or taxis from reaching the area to carry away the wounded and discouraged passersby from offering assistance.

In a classified report that could pose some awkward problems for U.S. policymakers, the FBI tentatively has pinned responsibility for the blasts, and the subsequent interference, on personal bodyguard forces employed by Hun Sen, one of Cambodia's two prime ministers, according to four U.S. government sources familiar with its contents.

The preliminary report was based on a two-month investigation by FBI agents sent here under a federal law giving the bureau jurisdiction whenever a U.S. citizen is injured by terrorism. But the FBI agents were forced to leave last month before they could complete their probe because U.S. Ambassador Kenneth Quinn told them they had been targeted for assassination and could not be protected adequately, the sources said.

The bureau says its investigation is continuing, but the agents involved reportedly have complained that additional informants here are too frightened to come forward.

Hun Sen, 46, is the most powerful man in Cambodia today, with a military force of about 1,000 men at his personal disposal, and diplomats here say that even if the charges are proven, he will not leave office without a fight. The chance of obtaining a fair trial for those involved in the bombing is also considered slim, because Hun Sen's party controls both the Interior Ministry and the judiciary.

That leaves Washington with few viable options for resolving the bombing case without destabilizing the Cambodian government.

"If Hun Sen ordered that act and the murder of more than 15 people, we want to know," a senior State Department official said. "As a practical matter of how you proceed {if it is proven}, I don't know." Another senior U.S. official predicted that Washington will have enormous difficulty ending its dealings with Hun Sen, given a lack of viable political alternatives "if you want to get something done in Cambodia."

On June 17, troops allied with Hun Sen skirmished in the capital with those allied with the other co-prime minister, Prince Norodom Ranariddh, who has accused Hun Sen of orchestrating the bombing. A rocket-propelled grenade landed near Quinn's residence. The fighting was quickly squelched, but the capital has remained on edge.

The two prime ministers also have been jockeying over whether new defectors from the Khmer Rouge guerrilla group led by Pol Pot are to be given a voice in national politics, amplifying tensions here and causing some observers to fear the country may soon be plunged back into civil warfare.

The organizer of the March 30 protest, Sam Rainsy, 48, leader of the chief opposition Khmer National Party, escaped injury in the attack when one of his bodyguards shoved him to the ground and stepped in the path of a blast. Rainsy has called for an international commission to investigate the blast and ensure that those behind it are punished.

"Finding the attackers . . . is critical to the process of ensuring the rule of law and rooting democracy in Cambodia," he said.

No international intervention is needed, countered Interior Minister Sar Kheng, a high-ranking official of Hun Sen's political party. Speaking as a law enforcement official, he said the local investigation "is going smoothly but not fast," and pledged to pursue the allegations by arranging for his investigators to interview officers of Hun Sen's bodyguard force -- with Hun Sen's permission.

"Now let me speak as a party official," Sar Kheng went on. "We did not commit this crime although we are blamed." He noted that the incident had greatly benefited Rainsy's party by showering him with publicity and sympathy.

When the demonstrators arrived at a grassy park in front of the assembly building on March 30, carrying a banner that said, "Down with the Communist Judiciary," they found the area surrounded by troops dressed in the distinctive battle gear of the unit assigned to protect Hun Sen, whose barracks were several blocks away.

Rainsy said in an interview that he spoke for 10 minutes, standing on a stool, and the first grenade was lobbed at him a minute after he stepped down. Then "someone said, We missed the target,' and another grenade landed exactly where I had been standing. . . . A third grenade came {six feet} in front of me. They were following me as I crawled away."

Abney, an official of the International Republican Institute who was helping Rainsy's party organize, said he was walking over to shake Rainsy's hand when shrapnel struck his leg "like a bottle or a brick" and knocked him to the ground. "Whoever threw them must have done so in full view of the troops," he said. "They disappeared into a pagoda, right in front of the compound for Hun Sen's troops."

One woman told interviewers at the Cambodian League for the Promotion and Defense of Human Rights that she appealed to some soldiers for assistance after shrapnel hit her hand. "Instead of helping, they hit me on my head," she said. Another witness told U.N. investigators that some of the soldiers urged a suspected grenade-thrower to "run faster" to escape pursuers from the crowd.

Ly Thuch, a senior adviser to Ranariddh and cabinet director for his party, cited the incident as a sign that national elections slated for next year "will be very hot. The intimidation is starting already." Similarly, a U.N. official said, "There has been a gradual recourse by the government to violence to suppress any form of political debate. . . . I see this grenade attack as a turning point." Staff writer Brian Duffy contributed to this report in Washington.