Raymond Clark III is accused of strangling Annie Le, of Placerville, Calif., in September, just days before she was to be married.
Prosecutors said in New Haven Superior Court on Tuesday that the new felony murder count had been filed against Clark. Both murder and felony murder carry 25 to 60 years in prison on conviction.
Felony murder is alleged when someone dies during the commission of a felony or attempted felony. Under Connecticut's felony murder law, prosecutors don't have to prove that a killing was intentional.
New Haven defense lawyer Hugh Keefe, who is not involved in the case, said the charge gives a jury another option if it finds that a homicide wasn't intentional.
Details of the new charge, including what the alleged felony was, were not released. Clark's lawyer, Beth Merkin, declined to comment on specifics of it.
Le vanished Sept. 8 from the Yale medical school research building where she and Clark worked, and her body was found five days later, on what was to be her wedding day.
The motive for the killing remains unclear.
An arrest warrant affidavit alleges Clark raised suspicions when he began scrubbing floors after the crime and tried to move a box of bloody wipes from the view of an investigator.
Authorities wrote in the warrant that a green-ink pen found under Le's body had her blood on it, as well as DNA from Clark on its cap. Police have said Clark signed into the secure building with a green pen on Sept. 8, the day Le disappeared.
The warrant says DNA from both Le and Clark was on a bloody sock found hidden in a ceiling.
The document also says Clark moved a box of wipes to hide blood spatters on it. Clark had a scratch on his face and left biceps that he said came from a cat, the warrant says.
Clark, 24, remains jailed in lieu of $3 million bail.
