HOPKINTON, Mass. (AP) – The same small-caliber bullet appears to have killed a mother and her 9-month-old daughter who were found dead together at home in the same bed, in what police are calling the first homicide in this Boston suburb in more than a decade.

When officers first found the bodies of Rachel Entwistle, 27, and her daughter, Lillian Entwistle, on Sunday, they thought that they might have been the victims of carbon monoxide poisoning, Middlesex District Attorney Martha Coakley said Monday.

There was no obvious blood on the bed where they were found, and the pair was fully clothed and tucked beneath the covers in the master bedroom, Coakley said.

It was not until medical technicians found the bullet wounds in the torso of both mother and child that police suspected foul play. Austopsies are scheduled for Tuesdsay.

No one had heard from the Entwistles since Thursday night, and it is not clear how long they had been dead.

Investigators are trying to find Rachel’s husband, Neil Entwistle, 27, who has been out of town since at least Friday and may be driving a BMW, Coakley said. She called Neil Entwistle a “person of interest,” as any family member would be at this point in the investigation, but stopped short of calling him a suspect.

“I’m not going to label anybody a suspect at this point,” Coakley said.

Relatives told police that they last spoke with Rachel Entwistle on Thursday night and had normal conversation.

The couple had organized a dinner party at their home on Saturday night, but when guests arrived no one answered the door and the house was dark, Coakley said.

“We can speculate that as of Saturday they were probably dead when people showed up at the home,” Coakley said.

Relatives got worried and called police on Sunday night. Officers found the bodies at about 6:30 p.m. An autopsy was scheduled for Tuesday.

The doors to the house were locked when officers found the bodies, and there were no signs of forced entry, police said, leading investigators to believe that the killings were not random.

“This is a quiet, professional neighborhood,” said Hopkinton Police Chief Thomas Irvin. “It’s generally very safe.”

The Entwistles moved 10 days ago to their two-story, colonial-style house on a wooded cul-du-sac. They had been previously living with relatives in southeastern Massachusetts, Coakley said, but she would not be more specific.

The couple married in August 2003 and relatives told investigators that the Entwistles did not have a history of marital problems. He worked in technology and was looking for a job; she had been a teacher but also was not working, Coakley said.

Neither Rachel nor Neil Entwistle has been arrested in Massachusetts, has any a record of local restraining orders, or has other telltale signs of domestic violence, Coakley said.

The last time Hopkinton police investigated a homicide was in February 1995, when Marta Santiago, 30, of Framingham was found stabbed to death on a secluded dirt road. Domingo S. Ardon, then about 40, was convicted in the case and sentenced to life in prison without parole.

Since then the rural town of 14,000 people about 35 miles west of Boston has had very little violent crime.

Late Monday afternoon a police cruiser blocked the Entwistles’ cul-du-sac. Kenny Ritchie, 17, stopped his car to chat with a cluster of reporters.

“That’s insane to think that something like this could happen in a small town like Hopkinton,” Ritchie said, when he was told the news. “Nothing really ever happens here … it’s kind of freaky.”