LOWELL, Mass. — Seven people died early this morning in a fast-moving apartment fire, officials said.

All seven victims were found in third-floor units of the three-story building that had businesses on the ground floor and apartments on the upper floors, fire officials said.

“It’s a tragic day for the city of Lowell,” Mayor Rodney Elliott said.

A police officer on patrol was the first to report the fire just before 4 a.m., while several tenants ran about 100 yards down the street to the nearest fire station to sound the alarm, Pitta said. But the building was fully involved by the time firefighters arrived. The fire eventually went to three alarms.

The cause and origin of the blaze remain under investigation, State Fire Marshal Stephen Coan said.

The building did not have a sprinkler system, but was not required to, Lowell Fire Chief Edward Pitta said. It did have an alarm system and whether that was working will be part of the investigation.

Authorities say 48 people lived in the building, which sustained heavy damage.

Several people had to be rescued from upper floors and taken to the hospital.

Witnesses said tenants were jumping out of windows.