KABUL, Afghanistan — International military forces worked today to recover every last piece of a Chinook helicopter that crashed over the weekend, killing 30 American troops, seven Afghan soldiers and an Afghan interpreter, NATO said.

German Brig. Gen. Carsten Jacobson, a spokesman for the U.S.-led coalition, told reporters that troops had secured the crash site in a rugged area of eastern Wardak province and nobody was being allowed in or out of the area while the investigation was ongoing.

Jacobson said the coalition still had not yet determined the exact cause of the crash, but some officials have said the heavy and lumbering transport helicopter was apparently shot down. Officials said the helicopter was hit as it was flying in and approaching the area.

“We are still investigating this incident so we have no picture of what was the cause for the incident. That is what the investigation is basically all about,” Jacobson said.

The helicopter was ferrying a group of U.S. Navy SEALs to reinforce a group of U.S. Army Rangers who were under fire. It remains unclear if the Rangers and SEALs were taking part in a night raid to capture or kill an insurgent leader.

It was deadliest single incident for U.S. forces in the decade-long war.

Another NATO helicopter made a hard landing today in eastern Paktia province, the U.S.-led coalition said. It did not report any casualties and said the cause of the hard landing was under investigation. The coalition said there was no enemy activity in the area at the time.

The helicopter was a CH-47 of the same type that crashed on Saturday. It was flying in to pick up special operations troops, but apparently suffered a mechanical failure and crash-landed, an officer in the war zone said.

The fatal crash on Saturday highlights the risks confronting the U.S.-led coalition as it looks to rely more on special operations forces while reducing the overall number of troops in Afghanistan by the end of 2014.

A current and a former U.S. official said the Americans included 22 SEALs, three Air Force members and a dog handler and his dog.

All but two of the SEALs were from SEAL Team 6, the unit that killed Osama bin Laden in Pakistan last May, officials said. None of the SEALs killed in the crash took part in the bin Laden mission.

The Rangers, special operations forces who work regularly with the SEALs, secured the crash site in the Tangi Joy Zarin area of Wardak province, about 60 miles southwest of Kabul.

Eight Taliban fighters were also reportedly killed in the battle.