Philip Markoff, 24, was found unresponsive in his cell Sunday morning in the Nashua Street Jail, the Suffolk County district attorney's office said in an e-mailed statement, and he was pronounced dead at about 10:15 a.m.
"Markoff was alone in his cell, and all evidence collected thus far indicates that he took his own life," the statement said.
Authorities will investigate to determine the facts and circumstances surrounding his death, the district attorney said.
Saturday would have been Markoff's first wedding anniversary, but his nuptials were canceled after his arrest.
Markoff, a former Boston University student, pleaded not guilty in the fatal shooting of Julissa Brisman, of New York City, and the armed robbery of a Las Vegas woman. Both crimes happened at Boston hotels within the span of four days in April 2009. Rhode Island prosecutors also accused him of attacking a stripper that week.
His trial in the Massachusetts cases was expected in March.
Markoff's lawyer, John Salsberg, said he was shocked and saddened about his client's death. He would not comment further.
Markoff had met the women through advertisements for erotic services posted on Craigslist, a classified advertising Web site, prosecutors said.
The Boston Police Department crime lab identified two blood stains taken from swabs on a handgun that was seized during a search of Markoff's apartment in Quincy, Mass., said prosecutors, who alleged Markoff used the weapon to bludgeon Brisman before she was shot three times at close range.
Investigators also found several other items in the apartment, including four pairs of women's underwear wrapped inside socks and hidden in a box spring, authorities said.
Markoff was engaged at the time of his arrest. His fiancee, Megan McAllister, ended the relationship with Markoff after visiting him in jail, and their wedding, scheduled for Aug. 14, 2009, was canceled.
