Yarnell fire hotshot deaths: Investigation begins 2 July 2013 Last updated at 17:14 Authorities have launched an investigation into the wildfire that killed 19 Arizona firefighters. The Yarnell Hill fire is burning out of control 80 miles (130km) north-west of Phoenix, having scorched 8,400 acres (3,400 ha). The dead were members of an elite "hotshot" unit who had fought fires across the south-western US. State and federal investigators expect to begin their inquiry on Tuesday by visiting the scene of the fire. As of Tuesday morning, the Yarnell Hill fire was holding in size but remained totally uncontained. About 450 firefighters battled the blaze, which has destroyed 50 buildings, threatened hundreds more and forced hundreds of people to evacuate. "This is a very difficult fire to fight," said emergency co-ordinator Karen Takai, noting vegetation on the ground was dry and oily. She told reporters firefighters currently battling the blaze were extremely well-trained to deal with the emotions of fighting a fire that killed a fellow crew. They acknowledge the sadness. You have to acknowledge it. You can't push it behind in your head," Ms Takai said. Acknowledge it and then get your head back in the game. They have to focus very hard on the ground. Memorial held The bodies of the deceased firefighters, aged 21-43, were received by an honour guard on Monday as they arrived in Phoenix for post-mortem examinations. And more than 1,000 people packed a memorial service for the firefighters later that evening in a university gymnasium in Prescott, Arizona, where the team was based. Prescott Fire Chief Dan Fraijo told mourners about a picnic he had recently had to welcome the department's new recruits and their families. "About five hours ago, I met those same families at an auditorium," he said. Those families lost. The Prescott Fire Department lost. The city of Prescott lost, the state of Arizona and the nation lost. Fire management official Clay Templin said the team, known as the Granite Mountain Hotshot crew, had been following safety protocols amid wind gusts that fanned the wildfire from 200 to 2,000 acres within hours on Sunday. The Granite Mountain Hotshot crew had spent recent weeks fighting wildfires in New Mexico and Arizona before being called to the Yarnell fire. Over the weekend, they entered the dry and smoky wildland with chainsaws and other heavy gear, where they set about removing brush and trees in order to halt the fire's advance . Heat wave havoc What is a dry thunderstorm, such as the one that sparked the Yarnell fire? Some time after 15:00 local time (22:00 GMT) on Sunday, in a last-ditch attempt to survive, the firefighters covered themselves with foil-lined, heat-resistant shelters as the flames passed over them. Only one member of the 20-person crew survived, because he was moving the unit's lorry at the time. Federal investigators from the Atlanta branch of the National Incident Management Organization arrived on Monday and were briefed in Phoenix. Judith Downing, a spokeswoman for the group, said state and federal officials would go to the scene on Tuesday. A fire prevention officer with the state's forestry department told the Arizona Republic newspaper investigators will review Sunday's weather conditions, fire department records, radio logs and any other evidence that would shed light on how to prevent a similar tragedy. "It will be designed so we can learn from this and teach up-and-coming firefighters if there are any lessons that can be learned," Carrie Dennett told the newspaper. Officials hope to have a preliminary report in the coming days. A separate investigation by the Maricopa County coroner's office will determine the firefighters' cause of death. The fire comes amid a scorching heat wave throughout the weekend across the western US. Dozens of people have been treated for exhaustion and dehydration. Temperatures in desert areas have reached 54C (130F), close to the world's all-time high, recorded 100 years ago in California's Death Valley.