Lockwood Fire Chief Bill Rash said the fire was reported just after 4 a.m., and firefighters arrived within 7 minutes, the Billings Gazette reported. Rash said firefighters had the blaze under control within 10 minutes.
Four bodies were found inside the trailer.
While Rash said there were no signs of arson, Yellowstone County Sheriff Mike Linder said the property would be treated as a crime scene until the cause is determined. The federal Bureau of Alcohol, Tobacco and Firearms joined the investigation.
Troy Donges, who lives across the street, said his son, who was up late listening to music, saw the fire and woke him. Donges rose quickly and ran outside.
"I was hitting the trailer as hard as I could with my hand," Donges said. "There was so much smoke in there, I don't know how anybody could've survived. People told me every time I hit it, it sounded like a gunshot and it woke them up."
Sheriff's Capt Bill Michaelis said the identities of the dead will not be released until investigators are sure all relatives have been notified.
Lockwood is a fast-growing suburb of Billings, Montana's largest city.