The three other people were taken to a hospital, District Attorney Michael Morrissey said during a press conference at the scene aired by WFXT-TV Sunday morning.
Two victims suffered serious but non-life-threatening injuries that appeared to be gunshot wounds, and one was transported for other medical reasons that also were not life-threatening, Morrissey said.
The shooting appears to have followed a party or event that took place inside the building, officials said.
No arrests had been reported as of Sunday evening.
When police arrived at the scene, they found people in the parking lot, including one person who needed help, which was rendered immediately, Morrissey said. That man was later identified as Small, officials said. More officers were then dispatched to the scene.
Police are investigating whether there was an event held inside one of the apartments that led to the shooting, Morrissey said.
“I believe that we’re looking at whether or not there was an event indoors that spilled outside,” he said.
An unknown number of people were evacuated from the apartment, he said. Police were working to get them back to their homes.
Morrissey said officers have to look at several areas both “inside the dwelling and in the parking lot area.”
Weymouth Police Chief Richard Fuller also spoke at the press conference and said there was no danger to the public.
He said several people may have left the area during or shortly after the shooting and urged them to call police at 781-335-1212.
Fuller praised the officers who responded during the frigid winter weather.
“It’s been an extremely cold night,” he said. “When I arrived here at 3:30 this morning, it was 8 degrees, so I have to commend the officers of the Weymouth police and the surrounding towns that responded.”
Police from Hingham, Abington, and Braintree responded to the scene, Traub, the Norfolk district attorney’s spokesman, said in a phone call later Sunday morning.
Twenty-six SWAT members and eight support officers representing a dozen area communities from the Metropolitan Law Enforcement Council also responded, he said.
Globe correspondent Abigail Feldman contributed to this report.
Andrew Stanton can be reached at [email protected]. Follow him on Twitter @_andrewstanton.