BIDDEFORD — Maine State Police detectives are searching for four people in connection with a shooting Sunday night that left two men dead of multiple gunshot wounds.

Neighbors described hearing a series of shots at 7 p.m., then seeing one man fall to the ground on the grass behind the building at 19 Western Ave. Two women and two men had been with the victims inside the two-story apartment house, but ran away after the shootings, state police said.

Lt. Brian McDonough, head of the state police Major Crimes Unit for southern Maine, said at a news conference Monday that police have leads on who the four people are, but had not located them by late Monday.

Police said Russell Lavoie, 42, of Old Orchard Beach and Jeffrey Lude, 37, of Biddeford both died of multiple gunshot wounds. One man died at Maine Medical Center in Portland and the second died in an ambulance, police said. Police did not say which man died where. They were unable to interview the men before they died.

Neither Lavoie nor Lude lived at the Western Avenue building, but their shootings were not a random act of violence, McDonough said. The two had a reason for being in an apartment in the building, and that ultimately led to the shooting, he said, but he would not elaborate. He said the shooting occurred inside that apartment.

Police would not say whether a gun was recovered, or whether they found evidence of criminal activity in the apartment. Police cordoned off the building and nearby property as part of their effort to locate possible trace evidence related to the crime.

A woman who said she was Lude’s mother came to the scene Monday and spoke with a detective, but said she was not ready to speak publicly.

Moises Cruz III, a neighbor, said he heard muffled shots Sunday night that he believed came from inside an apartment.

“I heard six, seven, eight shots. The last two were louder,” he said. “I saw a heavyset dude running and then it looked like he fell down.”

Cruz saw other people try to resuscitate the man, but without success, he said.

Western Avenue is a quiet street with a mix of single-family and multi-unit buildings just off outer Main Street. A steady stream of cars and pedestrians slowed as they passed the shooting scene early Monday.

Cruz said the apartment building where the shooting occurred was quiet, but there were frequently people coming and going. He said he complained at one point because the visitors parked on the side of the road in front of his building, making it harder for him to find a parking spot. The situation improved for a time, but then became a problem again, he said.

David Dunn, who owns the three-unit building, said he had trouble with people in the apartment where the shooting occurred.

One tenant who had been there for several years had not been a problem, but then became sick and invited an out-of-state female relative to live with him. The relative brought male friends from her home state who generated complaints, he said. Dunn said he believed the relative may have come from Massachusetts.

Dunn said he ordered the men to leave, out of concern that there may have been drug dealing taking place at the property. He said that as far as he knew, they had vacated the property.

Both of the men who were killed had criminal histories.

Lavoie had 11 convictions dating to 1990, including several for burglary and theft, drug trafficking in 2011, terrorizing in 2004 and assault in 2003, according to the State Bureau of Identification.

Lude’s record shows he was charged with failing to submit to detention in 2013, violating bail conditions in 2011, driving after his license was revoked as a habitual offender in 2004, drug trafficking in 2004 and assault in 1999.

State police have jurisdiction to investigate most homicides in the state, and Biddeford police have left state authorities in charge of releasing information to the public. A call to Biddeford police about previous calls for service at the apartment was not returned Monday. A spokesman for the Maine Drug Enforcement Agency said it did not have any record of drug activity at the apartment building.

McDonough said police were told who was in the apartment where the shooting occurred by someone who knew the people, but he would not elaborate. A man matching the tenant’s description was interviewed by police, neighbors said.

The Office of Chief Medical Examiner performed an autopsy on one of the slain men Monday and was planning to perform the second autopsy Tuesday.

Police spent Sunday night and Monday searching the apartment and the building’s grounds for clues. At one point Sunday night, police tried to track at least one of the suspects across nearby West Cutts Street.

Police also used lasers to map the scene, which could help determine bullet trajectories.

On Monday, a half-dozen orange cones marked the spot where at least one of the men fell, said neighbor Priscilla York. She lives across the street and said her 14-year-old grandson and 16-year-old granddaughter were outside when the shooting occurred.

“I thought it was fireworks. Next thing I know (my grandchildren) are saying, ‘Memere, that was gunshots,’ ” York said.

York said her grandchildren saw a group of five – four women and one man – scatter from the yard after the shots were fired. The grandchildren and a friend later provided statements to police, she said.

“There was a body where the cones on the grass are,” York said, pointing to the cones at the edge of the building’s driveway.

York said the people who lived in the building were generally quiet, as is the neighborhood.

“They seemed like decent people. There’s never any trouble,” she said. “You wouldn’t expect anything like this.”

Goliath Vanalehen, who lives in one of the other apartments in the building, said Monday that he had been grocery shopping when the shooting happened, but arrived home about 20 minutes later. He said he saw one man lying on the ground near the building.

“I’m surprised by this. Most of the people around here have young kids,” he said.

Vanalehen, who moved into the building a few months ago and didn’t know his neighbors, said he was worried about his dog, cat and chinchilla because they had been locked inside the apartment since Sunday night and he had been barred from entering the building. Police assured him the pets were being looked after and planned to let him in early Monday evening.