Taliban fighters shot at a US helicopter head-on from nearly point-blank range, US lawmakers heard Thursday, in what was the single deadliest incident for US and NATO forces in the Afghan war.

Lawmakers were grilling US military officials over the August 6, 2011 attack on the Chinook that killed 30 Americans as it transported Navy SEAL commandos, along with other American and Afghan troops, to flush out a Taliban commander in Wardak province.

Families of some of the victims have alleged the military has not revealed all the facts of the incident, failed to punish commanders in charge of the operation and mishandled some of the remains of the dead.

They have also suspected Afghan soldiers involved in the operation could have passed word to the Taliban of the chopper's route.

But officials sought to counter allegations that have circulated online, saying officers at the time employed the right helicopter and the right tactics and that the remains of the dead were handled with respect.

Taliban fighters atop a building near the designated landing zone fired rocket-propelled grenades (RPGs) at the chopper from nearly head-on, at a distance of less than 250 yards (meters), said Garry Reid, the Pentagon's principal deputy assistant secretary of defense for special operations and low-intensity conflict.

The close range left "the pilot no chance to perform evasive maneuvers," Reid said.

The attack occurred when the American CH-47D Chinook with the call sign "Extortion 17" flew low into the Tangi Valley.

US Army Ranger forces had staged an operation earlier targeting the Taliban figure and the Navy SEALs were flown in to try to cut off the escape route of the fleeing commander.

Reid dismissed the possibility that Taliban militants learned about the helicopter's route beforehand, saying only the Navy SEALs and air crew knew the flight route and landing zone.

A C-130 aircraft, a Predator drone and two Apache helicopters had flown over the designated landing area minutes earlier and failed to detect the Taliban fighters nearby, he said.

The harsh reality is that helicopters remain vulnerable to RPGs and other shoulder-launched weapons, he added, saying the Chinook is the standard helicopter used by troops at high elevations in Afghanistan due to its longer range and that flying a modified model designed for special forces would not have made a difference.

Chinooks have been shot down by RPGs previously in Afghanistan in 2005 and in 2002, he said before the House Oversight and Government Reform subcommittee on national security.

He told lawmakers "the fact remains we will always have to balance the tactical requirement to move troops quickly across the battlefield with the dangers of incurring lethal enemy fire and flying in extreme terrain."

Even the hearing itself was the subject of controversy, as some relatives reportedly complained that no commanders involved with the operation testified and the families were not invited to speak before the panel.

But not all the families wanted the hearing to take place and do not support the allegations made by some relatives, said John Tierney, a Democrat on the panel.

"They've asked for privacy and they seek closure."