Updated

Twin 4-year-old boys and their 5-year-old sister died in a house fire this week, and their mother is in custody, authorities in suburban Milwaukee said.

No adult was home when the fire broke out Thursday afternoon at the two-story house in West Allis, acting police Chief Charles Padgett said at a news conference.

He declined to give specifics about the mother's arrest, saying only that she was being investigated as a result of the overall circumstances. He added that she had been cooperating with the investigation.

"We're still trying to determine the exact circumstances of this incident and the cause and origin of the fire," he said.

The names of the mother and children have not been released, in part because authorities said they were still trying to locate the children's father.

"The City of West Allis is mourning this loss of life," West Allis Mayor Dan Devine said, "and the mourning is intensified by the fact that they were children."

Police blocked off half the street around the home Friday. The smell of burnt wood lingered in the air, and passers-by stopped and pointed at the yellow brick house, where the fire ate through the roof.

Property records say the home is owned by Todd Brunner of nearby Pewaukee. But Brunner told The Associated Press the house had been foreclosed upon about two years ago.

"I don't have anything to do with managing it. I just found out 15 minutes ago about the fire," he told AP on Friday afternoon. "I did not put the tenants in there and I didn't know them."

Agents from the federal Bureau of Alcohol, Tobacco, Firearms and Explosives and the state Department of Justice arrived Friday. Padgett said it was standard procedure for his department to involve federal officials for fire investigations. He declined to say whether authorities suspect foul play.

Interim Fire Chief Gary Streicher said firefighters fought the blaze aggressively but were stymied by a hole in the floor of the second-floor bedroom where the children were found.

"It's very difficult, as you can imagine," he said. "It's always tough to lose a life, and particularly when you're dealing with such young children it makes it doubly as bad."

The blaze marked the second fatal fire in a month in West Allis, a city with about 61,000 residents. A 13-year-old boy rescued his 4-year-old brother from a burning duplex last month; another brother and a friend died.