Seven people were killed in a three-alarm fire that hit a residential building Lowell, Mass., early Thursday morning.

The fire was discovered shortly before 4 a.m. by a police officer who was on patrol, according to MyFoxBoston.com.

Authorities said the bodies of four adults and three children were found in two different units on the top floor of the three-story building in the city, which located about 25 miles from downtown Boston. The building houses a liquor store and an apartment unit on the first floor, and held four apartment units each on the second and third floors.

Fire officials said they believe some 48 people were inside the building when the fire started. The fire prompted some tenants to jump or hand their children to safety.

Middlesex District Attorney Marian Ryan said it will take some time to identify the victims.

"It's a tragic day for the city of Lowell," Mayor Rodney Elliott said.

Lowell Fire Chief Edward Pitta said the building didn't have a sprinkler system, but it wasn't required to have one. There is an alarm system, but investigators are checking if it was working.

Meanwhile, investigators are looking into reports that the sound of fireworks exploding was heard before the fire started. It was not yet determined how the blaze started.

Residents in Massachusetts are barred by law to have fireworks in their possession.

The American Red Cross is assisting displaced tenants and the city is accepting donations of clothing and other essentials, Elliott said, while a relief fund has been set up at the Jeanne D'Arc Credit Union in the city.