Five men who appear to be from Mexico, worked together at a Chinese restaurant and died in a weekend fire were found in the basement of a Detroit-area home owned by their boss and likely were overcome by thick smoke, authorities said Monday.

The cause of the fire Sunday could be a mattress ignited by careless smoking, said David Molloy, public safety director in Novi, 20 miles northwest of Detroit.

The basement had stairs to the first floor, but the basement windows were made of glass block, which would prevent any escape in a sudden emergency.

"Visibility was zero. The way the fire was developing, they were likely overcome by the smoke and couldn't make it" up the stairs, he said.

Autopsies and efforts to identify the men were ongoing. Molloy said the victims appeared to be from Mexico, based on documents and evidence found in the basement.

"It's a very sad tragedy," he said.

The victims, in their 20s and 30s, worked at Kim's Garden, a nearby Chinese restaurant. The eatery's owner also owned the house although he didn't regularly stay there, Molloy said.

"By all accounts, these gentlemen had the ability to come and go. ... Witnesses said they would see them come out occasionally for a cigarette," the chief said.

The fire occurred in a comfortable neighborhood where some houses are worth more than $350,000.

He said the homeowner made a call to 911 after arriving at the home around 9:30 a.m. Sunday.

"We have talked to him once," Molloy said. "We are looking to talk to him to connect the dots."

Molloy said investigators are getting help from federal authorities on the immigration status of the five men.

"The investigation is in the infancy stage," he said.