The 19-year-old accused of gunning down three people at a party in Mukilteo, Washington, last weekend told police he did so because he was upset over a recent breakup. The suspect, identified as Allen Ivanov, is said to have confessed to the killings after being arrested by state troopers 100 miles from the Seattle suburb. Police said he told them he attacked because his ex-girlfriend, Anna Bui, seemed to be dealing with the couple’s breakup better than he was. Bui was one of the three killed. A probable-cause statement made public Monday also revealed Ivanov purchased an AR-15 semi-automatic rifle, which police identified as the murder weapon, shortly before the shootings. He sat in his parked car right across from the party to read the gun’s instruction manual right before the massacre, documents show. Bui and Ivanov were both students at the University of Washington. Ivanov had previously described Bui as his “dream girl,” but had broken up with her before seeking a reconciliation a few weeks before the shooting. The pair had spent time together, but Ivanov became upset when he found out Bui had been dating other people, police said.