PRESCOTT, Ariz. - A three-month investigation into the June deaths of 19 Arizona firefighters found that the men ceased radio communication for a half hour before they were killed in a wildfire blaze, but did not assign fault. Some family members say that reluctance could put other lives in danger.

The 120-page report released Saturday found that proper procedure was followed in the worst firefighting tragedy since Sept. 11, 2001. Investigators suggested that the state of Arizona should possibly update its guidelines and look into better tracking technology.

All but one member of the Granite Mountain Hotshots crew died June 30 while protecting the small former gold rush town of Yarnell, about 80 miles northwest of Phoenix, from an erratic, lightning-sparked wildfire. Hotshots are elite backcountry firefighters who hike deep into the brush to fight blazes.

While maintaining a neutral tone, the investigation cited badly programmed radios, vague updates, and a 33-minute communication blackout while the men hiked out of their safe zone to the spot where they would eventually be overcome by the fire. Though the report points to multiple failures, investigators did not consider whether the deaths could have been avoided, raising questions about what lessons firefighters will be able to take from the tragedy.

At a news conference in Prescott, where the fallen firefighters lived, Shari Turbyfill implored officials to draw stronger conclusions about why her stepson and his comrades died, and recommend immediate changes.

"Your protection of us is killing us," she said. "We're willing to take the heat right now, but I don't want another family to deal with this."

Her husband, David, said the command center should never have lost track of his 27-year-old son, Travis.

"You have to look at communications and GPS devices," he said.

The report, produced by a team of local, state and federal fire experts, provides the first minute-to-minute account of the fatal afternoon. The day went according to routine in the boulder-strewn mountains until the wind shifted around 4 p.m., pushing a wall of fire that had been receding from the firefighters all day back toward them.

After that, the command center lost track of the 19 men. Without alerting headquarters, and despite the weather warning, the firefighters left the safety of a burned ridge and dropped into a densely vegetated basin surrounded by mountains on three sides. Investigators noted that the men failed to perceive the "excessive risk" of this move and said there was no way to know why the firefighters made the deadly decision. The crew, known for its aggressiveness, may have been headed to a fallback safety zone closer to their trucks, so that they could retackle the fire more quickly.

The command center believed the firefighters had decided to wait out the wind shift in the safety zone.

Command did not find out the men were surrounded by flames and fighting for their lives until five minutes before they deployed their emergency shelters, which was more than a half hour after the weather warning was issued.

Without guidance from the command center or their lookout, who had escaped after warning the crew, the men bushwhacked into a canyon that soon turned into a bowl of fire. The topography whipped up 70-foot flames that bent parallel and licked the ground, producing 2,000 degree heat.

As the flames overcame the men, a large air tanker was hovering above, trying to determine their location.

The firefighters may have failed to communicate during that crucial half hour because they entered a dead zone, or because they were wary of overloading the radio channels. In the end, the same communication gaps that stymied the rescue effort hindered the reconstruction of the tragedy.

"We don't know that information; we don't have it," lead investigator Jim Karels said. "That decision process went with those 19 men."

The Hotshots had said they were in "the black," which was taken to mean they were safe. It's not unusual for backcountry firefighters to go out of touch for chunks of time, and no one checked back with the crew.

Meanwhile, residents were evacuating and other crews were being pulled off the fire line, creating a "complex, busy, hectic situation," Karels said.

The investigators recommended that Arizona officials review their communications procedures and look into new technologies, including GPS. But they stopped short of saying the technology would have saved lives.

Ted Putnam, a former fire investigator for the U.S. Forest Service, said the report should have taken a harder look at the hotshot culture and the choices made by the crew.

"What I'm arguing really went wrong is in the decision area," he said.

When it began June 28, the fire caused little immediate concern because of its remote location and small size. But the blaze quickly grew into an inferno, burning swiftly across pine, juniper and scrub oak and through an area that hadn't experienced a significant wildfire in nearly 50 years.

The fire destroyed more than 100 homes and burned 13 square miles before it was fully contained on July 10.

No other wildfire had claimed the lives of more firefighters in 80 years.