Canyon County sheriffs arrived at the residence at 10:44 p.m. MST, where Eilers was found shot and wounded.
"Emergency medical staff took live-saving measures, but they were unsuccessful," said Capt. Dana Maxfield.
Eilers was pronounced dead at the scene. An autopsy conducted Friday released the cause of death as a single gunshot wound to the chest. The bullet reportedly punctured Eilers’ lung before exiting his body, said Monte Cox, Eiler's longtime manager.
Cox said Eilers had gathered with relatives for the holidays, but tensions escalated with an ex-girlfriend, also in attendance. Eilers was asked to leave the residence, but was said to have refused and began throwing dishes and other objects in protest. The home’s owner was then said to have pulled a gun on Eilers and shot him in the chest after a brief verbal confrontation.
Eilers’ ex-girlfriend, mother, and young son were said to have been in the room at the time of the shooting.
James Robert Malec, 48, was arrested and charged with second degree murder in relation with the shooting, said Capt. Maxfield. It is believed that Malec is Eilers’ mother’s boyfriend. Capt. Maxfield could not confirm this relationship or Eilers' alleged interaction with the ex-girlfriend. Maxfield did confirm that numerous relatives were at the scene at the time of the shooting.
Eilers was introduced to the fight game by close friend and former UFC champion Jens Pulver, who saw potential in the former Iowa State middle linebacker. After a brief tryout, Eilers became a member of the formidable Miletich Martial Arts team.
A well-liked figure in the fight community, the athletic heavyweight quickly rose through the ranks and fought for the UFC heavyweight championship against Andrei Arlovski at UFC 53 in June 2005. The fight was stopped 4:10 into the first round when Eilers incurred a knee injury and collapsed to the canvas.
Eilers (19-7-1) appeared in the Octagon four times, and went 1-3 in the UFC.
The Boise native reeled off 10 victories in his next 11 fights and got his second title bid for the EliteXC promotion in 2008.
After a second-round loss to the much heftier Antonio Silva last July, Eilers announced he’d be moving down to the 205-pound division.
“He was really getting his fight career together,” said Cox, who called Eilers a good friend.
Jeff Sherwood and Brian Knapp contributed to this report.
This article was updated at 4:10 p.m. EST to correct and add information pertaining to Eilers' death.