The crash of a Chinook helicopter that killed 38 U.S. and Afghan troops was probably caused by an insurgent firing a rocket-propelled grenade, U.S. military officials said Monday, describing the attack as a “lucky shot.”

Investigators intensified their probe into the biggest single loss of life for U.S. forces since the start of the Afghanistan war a decade ago. While military officials played down concerns that their workhorse helicopters had become more vulnerable to attacks by the Taliban, investigators focused on whether it had been necessary to dispatch the Chinook on the risky mission in the remote Tangi Valley of eastern Afghanistan.

NATO officials in Kabul said a ground team of Special Operations forces had been hunting a suspected Taliban leader in Wardak province early Saturday when they became engaged in a firefight with “several” insurgents in Sayyidabad district. Although some of the enemy fighters were killed, the U.S. forces requested reinforcement, NATO said in a statement released Monday.

A commando force of 22 Navy SEALs and three Air Force Special Operations personnel scrambled for a rescue mission and boarded the Chinook, which was operated by five Army aviators. The helicopter also carried eight Afghan soldiers.

The chopper crashed just as it was arriving on the scene, felled by a rocket-propelled grenade, NATO said. It is relatively rare for the Taliban to shoot down U.S. helicopters; nonetheless, the aircraft are prone to coming under small-arms or rocket fire when they take off or land.

Everyone aboard the Chinook perished, but the U.S. commandos on the ground did not suffer any casualties and were able to fend off the insurgents on their own. They also managed to secure the crash site until another helicopter arrived, military officials said, raising questions for investigators about whether the original rescue mission was necessary.

Military officials said the crash would not deter them from deploying Special Operations forces in aggressive night raids against Taliban fighters, a key part of the U.S. strategy in the conflict. They also said that the setback did not represent a broader Taliban resurgence.

“At this point, it’s a one-off incident,” said Marine Col. David Lapan, a Pentagon spokesman. “It’s the danger of operating aircraft in combat.”

In Tampa, Defense Secretary Leon E. Panetta, who was attending a change-of-command ceremony for the leadership of the U.S. Special Operations Command, praised the fallen troops and said the military would not back off in the fight.

“As heavy a loss as this was, it would even be more tragic if we allowed it to derail us from our efforts to defeat al-Qaeda and deny them a safe haven in Afghanistan,” Panetta said.

Remote areas such as the Tangi Valley, where the Chinook crashed, have long posed a dilemma for U.S. and Afghan forces. From 2006 to 2009, the United States established outposts in these enemy havens with the goal of driving off insurgents and winning the support of Afghans with jobs, roads and development.

By 2010, senior commanders had concluded that the strategy was not working and that the isolated valleys were not worth the effort to pacify. Throughout 2010 and early 2011, commanders shut down about a dozen small outposts in some of the most remote and hostile areas of eastern Afghanistan and shifted forces out of places such as the Tangi, Pech and Korengal valleys and into areas they deemed more strategically important to the war effort. Typically these were places with more people, better roads and more vibrant economies.

The decision to shut down established bases allowed the insurgency free rein, and, in recent weeks, U.S. forces have been drawn back into these areas. American commanders recently decided to reestablish a presence deep in the Pech Valley only six months after turning the area over to Afghan forces, who proved incapable of holding off the Taliban.

Similarly, the raid into the Tangi Valley represented an attempt to challenge the insurgents in an area where the United States had once had a presence but had chosen to pull out.

Senior commanders compare the raids in these remote areas to “mowing the grass.” Without a permanent presence, it is almost impossible for U.S. and Afghan troops to defeat the insurgency in these areas. The periodic raids are designed to prevent them from expanding to areas considered more crucial to the war effort.

U.S. forays back in these valleys can be especially deadly. The mountainous terrain allows the insurgents to mass troops on American positions. The lack of a permanent U.S. presence also allows them to build up firepower and establish fighting positions between raids.

As of late Monday, the Pentagon had not officially released the names of the 30 U.S. troops who died in the Chinook crash. But in the home towns of the fallen men, people came together to mourn and remember their friends, sons, husbands and fathers.

“We have had people in and out of the house all day, sharing our grief,” said Becky Harp, a cousin of Thomas Ratzlaff, a Navy SEAL master chief, who grew up in Green Forest, Ark. Ratzlaff was survived by two sons. His wife is expecting another baby in November.

“He wanted to be a Navy SEAL from the time he was in high school,” Harp said. “He had been in Afghanistan several times and said he felt he had to be there, he felt it was important work.”

Patrick Sasser, a childhood friend of Stamford, Conn., resident Brian Bill, another SEAL killed in the crash, remembered him as adventurous and energetic. “Brian definitely had a leadership quality about him,” Sasser said. “When Brian spoke, people listened.”

Over the weekend, another high school friend passed the news to Sasser, who spread the word among their friends, prompting a flood of sorrow and pride — much of it posted on a Facebook page in Bill’s memory.

Staff writers Alice Fordham and Greg Jaffe contributed to this report.