Robert Brown|Deadly explosion at Colorado apartment building was set intentionally, investigators say

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A large explosion last week at an apartment building in western Colorado that killed an 18-year-old woman and left residents of six units displaced appears to have been set off intentionally, authorities said Wednesday.

The determination comes nearly a week after firefighters were dispatched at 7:15 a.m. local time Friday, April 5, to the apartment building located in the small Colorado town of Palisade, over 200 miles west of Denver, after the explosion ripped through the apartment building. A subsequent fire left one woman dead who authorities also identified Thursday as Kloey Weythman, 18, according to a joint news release from the Palisade police and fire departments.

Authorities identified 29-year-old Andrew Alderman as the man suspected of setting off the explosion. Alderman was found dead by suicide within hours of first-responders arriving at the scene, police said.

"We recognize this is a horrible tragedy and an immensely difficult time for Kloey Weythman’s loved ones and that so many questions remain unanswered," according to a joint statement from the police and fire departments. "This investigation remains active and ongoing as investigators are committed to figuring out the reason behind this senseless act."

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Displaced residents provided temporary housing by Red Cross

Firefighters arrived within minutes of the explosion Friday to find the apartment building in Mesa County already engulfed in flames, authorities said.

As crews battled a heavy fire that spread rapidly through an aging infrastructure, multiple other first-responder and law enforcement agencies were called for assistance at the scene, according to a news release.

The fire left Weythman dead and displaced several residents, all of whom have been provided temporary housing by the American Red Cross. The Town of Palisade said it has partnered with the Red Cross to collect gift cards and monetary donations for the victims of the fire.

A vigil is planned for Friday evening at a town park, where residents will gather to plant perennial flowers in honor of the victims of the explosion, according to the town's website.

Accelerant found at Alderman's apartment; motive unclear

Shortly after the fire was contained, investigators reportedly discovered "disturbing" videos posted on Alderman's social media pages in which he took responsibility for setting off the explosion.

Alderman's phone was tracked more than 160 miles east of Palisade to an area outside of Leadville, Colorado, where he was found dead around 4 p.m. in his car. The Lake County Sheriff’s Office determined that Alderman had died by suicide, according to the news release.

Detectives investigating the cause of the explosion and resulting fire found indications at the scene that Alderman used accelerant to set off the blaze at his own ground floor apartment. The explosion itself was caused by "a high volume of vapors from the accelerant within that apartment," authorities said.

Authorities found no evidence that the explosion was caused by a bomb or a drug lab and have also ruled out natural gas and appliance failure as the source. Additional lab results will determine the exact chemical accelerant that Alderman used, authorities said.

"At this point, we are unable to arrive at a definitive motive," the police and fire departments said in a joint statement. "Unfortunately, we may never know or be able to determine what caused the suspect to do what he did."

Eric Lagatta covers breaking and trending news for USA TODAY. Reach him at [email protected]