(CNN) -

Seven people -- including three children -- were killed and at least 10 other people were sent to a hospital as a result of an apartment fire Thursday morning in Lowell, Massachusetts, officials said.

The fire, reported just before 4 a.m., burned through some apartments above a liquor store, state Fire Marshal Stephen Coan told reporters.

Those who died were found in two of the four apartment units on the third floor, Coan said.

Investigators are looking into reports that people heard sounds of fireworks coming from the building around the time the fire started, but no determination about the fire's cause has been made, Coan said.

Authorities also are investigating reports that no alarm sounded in the building, even though the structure had a fire alarm system, Coan said.

"I expect this to be a long and thorough process," Middlesex County District Attorney Marian Ryan said of the investigation.

The building did not have a sprinkler system, but a structure that size would not require one, Lowell Fire Chief Edward Pitta said.

Some people jumped from windows, and firefighters rescued others with a ladder, Pitta said. The building generally had about 50 occupants, but investigators didn't know how many people were there when the fire began, he said.

Parts of the interior collapsed, and that will slow the investigation, Coan said.

Ten victims were being evaluated at Lowell General Hospital, hospital spokeswoman Angela Strunk said. Their ages and conditions weren't immediately available.

Lowell is about 35 miles northwest of downtown Boston.