Police and human rights activists headed to an isolated river town along the Honduran coast Thursday to investigate what happened last week during a gun battle that local officials say left four innocent people, including two pregnant women, dead in a drug bust orchestrated by U.S. agents.

Relatives began to come forward, trying to convince investigators — and the U.S. government — that those shot were not drug smugglers, but locals who use the rivers as roads and were moving from place to place when attacked in a raid by Honduran police flying in U.S. helicopters and aided by U.S. Drug Enforcement Administration agents.

“The boat was just transporting people and some cargo. They were coming to town to spend Mother’s Day,” said Serene Trapp, a cousin of Candelaria Trapp, 45, a pregnant single mother of six who was allegedly killed in a burst of gunfire from a helicopter. “I know all the people on the boat,” Serene Trapp said in a phone interview. “None of them were smugglers.”

News of the firefight, along with allegations of the four deaths, led local residents to torch several houses and demand that U.S. drug enforcement agents leave the area, according to the mayor.

The gun battle took place Friday along the muddy Patuca River, near the northeastern town of Ahuas, as Honduran forces assisted by U.S. advisers in four U.S. helicopters sought to seize a load of cocaine being moved from an illicit jungle airstrip to a waiting boat.

“First the narcos opened fire, and later the DEA helicopters were searching the area, and they fired with their guns at the boat with civilians, thinking maybe they were the narcos,” the mayor of Ahuas, Lucio Vaquedano, said in an interview.

Vaquedano said that several bodies had been recovered and that the boat the civilians were allegedly aboard was pocked with large bullet holes, which he said police told him were from a .50-caliber gun, the kind used by a door gunner on one of the U.S.-supplied helicopters. “It was easy to get confused because there were two boats, and the narco boat didn’t have lights and the civilian boat was running with its lights on,” he said.

Ahuas is a town of 1,500 people, many of them members of indigenous Miskito tribes, in the state of Gracias a Dios.

U.S. officials said Thursday that at least “several” DEA agents had served as advisers during the raid but that the American officers, while armed for self-defense, did not fire their weapons.

The U.S. officials, representing law enforcement agencies, and diplomats who have been briefed on the mission also cast doubt on the allegations that innocent people were killed during the 2 a.m. mission, though they said an investigation is ongoing. The U.S. officials said it was not unusual for local authorities to work with smugglers and also said they wondered why innocent civilians would be on the water in the middle of the night.

With congressional approval and in coordination with the State Department’s Narcotics Affairs Section, the DEA has sent advisory support teams to train and coordinate anti-drug operations with units of the Honduran National Police. These military-style fast-response units, which use U.S. intelligence, radar tracking of illicit flights and U.S. helicopters, seized 22 metric tons of cocaine last year — a record amount — but they have also generated controversy, as human rights advocates criticize the further militarization of the drug wars.

U.S. officials, speaking on the condition of anonymity because their statements had not been authorized, said the operation began early May 11 when civilian and law enforcement radar began tracking a suspicious flight into the area. The Honduran Mosquito Coast is now a top destination of drug flights in the region, according to DEA agents, who call the wild and mostly roadless area “red hot.”

According to the U.S. account, an illegal flight by a mid-size propeller plane landed at 1:30 a.m. and a flyover by helicopters with night-vision capabilities counted 30 men on the ground at a small airstrip off-loading bundles from the plane into a waiting pickup truck.

Ten minutes later, the truck arrived at the river and the men started to move bundles into a waiting boat. The helicopters landed, and the workers fled. Honduran and U.S. officials said the operation netted 14 bundles containing 450 kilograms — almost 1,000 pounds — of cocaine.

At 2:30 a.m., according to the same officials, another boat arrived and began firing at the Honduran police and DEA agents. One of the helicopters, which may have been on the ground, returned fire and the boat sped away, the officials said.

Melvin Duarte, a spokesman for the Honduran Public Security Ministry, said, “At this moment we cannot give out any information, as the police are preparing a report on the incident.”

Ruth Donaire, a spokeswoman for the National Police in Honduras, said a commission had been established to investigate the case but noted the difficulty of access to the area. “We cannot confirm the number of dead because we currently have four different versions of the facts, with four different figures,” she said.

U.S. and Honduran human rights activists demanded more information.

“It is critical that both Honduran and U.S. authorities ensure that the killings are thoroughly investigated to determine whether the use of lethal force was justified,” said Jose Miguel Vivanco, Americas director at Human Rights Watch, in a statement. “If evidence demonstrates that security forces violated international standards, they must be held accountable.”

Correspondent Nick Miroff and researcher Gabriela Martinez contributed to this report.