BOSTON — Deborah Hussey had no idea what she was in for when she stepped into the South Boston house that served as both death chamber and graveyard for some of the 19 people mob boss James “Whitey” Bulger is accused of murdering.

Her stepfather, and Bulger’s right-hand man, Stephen “The Rifleman” Flemmi, had a pretty good notion. Hussey, 26, was not the first person Flemmi saw Bulger murder in that house, he testified on Monday.

“She walked into the kitchen area, Jim Bulger stepped out from behind the top of the basement stairs and he grabbed her from behind and started strangling her,” Flemmi, now 79, told the court at Bulger’s trial on charges related to 19 murders.

Bulger lost his balance and the two toppled over, Flemmi continued, but the gangster’s grip was surer than his footing.

“It didn’t take long,” Flemmi said of the 1985 killing, which was the second time he watched Bulger strangle a young woman. “She was a very fragile woman.”

Once she stopped breathing, the mobsters followed their normal post-killing routine. Bulger went upstairs to take a nap while Flemmi and another gang member dug a grave in the basement.

Bulger faces the possibility of life in prison if convicted of charges related to 19 murders he is accused of committing or ordering in the 1970s and ’80s while running Boston’s Winter Hill gang.

The 83-year-old defendant has pleaded not guilty to all charges, although his attorney has admitted Bulger was a drug dealer, extortionist and loan shark, in other words an “organized criminal.”

On Friday, Flemmi testified about how Bulger had strangled Flemmi’s girlfriend, Debra Davis, four years before killing Hussey.

Both women were killed because Bulger believed they knew too much about the gang’s dealings and could expose them. Flemmi testified that Hussey was a drug user who had taken to hanging out at a bar the Winter Hill gang frequented. She was indiscriminately dropping their names to extract money from low-ranking gang members who feared her stepfather.

Flemmi is serving a life sentence for 10 murders he confessed to a decade ago. Unlike two other top Bulger associates who took the stand during the trial, now in its seventh week, he is still in prison.

He acknowledged under cross-examination that he had had a sexual relationship with Hussey.

“This little girl who would call you ‘Daddy’ is the same girl you would start abusing sexually a decade later?” defense attorney Henry Brennan asked Flemmi.

Flemmi responded that his sexual relationship with Hussey was consensual.

Flemmi said Bulger often took naps after murdering people.

“Maybe he was physically exhausted from it, I don’t know,” Flemmi said. “Maybe he would get high on it.”

While their boss rested, Flemmi and associate Kevin Weeks dug a hole in the dirt floor of the basement and buried Hussey’s body there. Flemmi said he first removed the woman’s clothes and some of her teeth, to make it harder to identify the body if it were found.

Hussey was not the first person the gang killed in the house and buried in its dirt basement floor, Flemmi recalled.

Two years before strangling Hussey, Bulger killed another gangster in the house, Flemmi said. He said Bulger shot Arthur “Bucky” Barrett in the head as he walked down the basement stairs with Bulger and Flemmi.

“I didn’t know he was going to shoot him, because I was right in front of him, in the line of fire,” Flemmi said. “The body hit me and we both fell down the stairs … I was angry with him [Bulger]. I said, ‘You could have shot me.’”

Flemmi recalled pulling Barrett’s teeth from his head with a pair of dental pliers.

“I only took a few out. I just couldn’t do it fully,” Flemmi said. He agreed with prosecutor Fred Wyshak that the extractions were a “gruesome task.”

Prosecutors say Bulger gave a corrupt FBI agent from his Irish-American neighborhood information about the Italian mafia, and that the agent in turn turned a blind eye to Bulger’s criminal activity. Bulger and his lawyers have vigorously denied the contention that he ever was an informant.

Witnesses at the trial have recalled an era when Bulger and his cronies routinely shook down bookies, drug dealers and business owners, threatening them with machine guns and burying victims in basements of houses and along the shore.

Bulger fled after a 1994 tip from a corrupt FBI agent that arrest was imminent. He was finally captured after 16 years on the run.