A 17-year-old girl attending a private party at an upscale Boston hotel last February was shot in the head and killed when she “swatted” the hand of a man holding a gun, prosecutors said Tuesday.The suspect, Messiah Leggett, 21, of Boston, was arraigned Monday on charges of involuntary manslaughter and unlawful possession of a firearm in the death of Nalijah Andrade, of New Bedford.Not guilty pleas were entered on Leggett's behalf and bail was set at $25,000.His attorney, Meg Stanley, said the investigation is ongoing.“It is too early for me to comment as it is a pending matter. Please keep an open mind until all the evidence is presented to the court as the case gets underway," she wrote in an email Tuesday. Andrade was among four juvenile girls who attended the party at the Hyatt Regency on Feb. 4, according to a statement from the office of Suffolk District Attorney Rachael Rollins.The partygoers were drinking and “taking selfies" with the 9mm semi-automatic pistol, authorities said.“Mr. Leggett was handling the gun in a playful manner when Ms. Andrade swatted at his hand, causing him to accidentally discharge a single round of ammunition," prosecutors said.Another man who allegedly handled the gun at the party also faces charges and is scheduled to be arraigned Oct. 5.