An apartment building fire killed at least four people Saturday morning on the city's North Side, authorities said.

One other man was critically injured when he jumped from a third floor window, said Fire Department spokesman Larry Langford.

"This is a bad one," Langford said.

Firefighters were on the scene within about two minutes after the fire was reported, just before 7 a.m.

The blaze was quickly extinguished, but when firefighters entered to search the building they found three people dead in one apartment and one dead in a second apartment, Langford said.

Investigators had not determined the cause of the blaze, he said.

The three-story brick building has about a dozen units and stands about a block from Wrigley Field.