A fourth-year student from South Korea was behind the massacre of at least 30 people locked inside a Virginia Tech campus building in the deadliest shooting rampage in modern US history, the university said Tuesday. Ballistics tests also show that one of the guns inside that building was used in another shooting two hours earlier, at a dormitory, Virginia State Police said. Police identified the shooter as Cho Seung-Hui, 23, a senior in the university's English department who lived on campus. Authorities said he was a legal resident of the United States. "It's certainly reasonable to assume that Cho was the shooter in both cases," but authorities have not made the link for sure, said Col. Steve Flaherty, superintendent of the Virginia State Police. A law enforcement official, speaking on condition of anonymity because the information had not been announced, said Cho was carrying a backpack that contained receipts for a March purchase of a Glock 9 mm pistol. Some students said their first warning came more than two hours after the first shooting, in an e-mail at 9:26 a.m. By then the second shooting had begun. "I think the university has blood on their hands because of their lack of action after the first incident," said Billy Bason, 18, who lives on the seventh floor of West Ambler Johnston, a high-rise coed dormitory where the shooting began and two people died. University president Charles Steger said the university was trying to notify students who were already on-campus, not those who were commuting in. "We warned the students that we thought were immediately impacted," he told CNN. "We felt that confining them to the classroom was how to keep them safest." He said investigators did not know there was a shooter loose on campus in the interval between the two shootings because the first could have been a murder-suicide. Two students told NBC television's "Today" show they were unaware of the dorm shooting when they reported to a German class where the gunman later opened fire. Derek O'Dell, his arm in a cast after being shot, described a shooter who fired away in "eerily silence" with "no specific target - just taking out anybody he could." After the gunman left the room, students could hear him shooting other people down the hall. O'Dell said he and other students barricaded the door so the shooter couldn't get back in - though he later tried. "After he couldn't get the door open he tried shooting it open... but the gunshots were blunted by the door," O'Dell said. The shooting began about 7:15 a.m. on the fourth floor of the dorm. Police were still investigating around 9:15 a.m., when a gunman wielding two handguns and carrying multiple clips of ammunition stormed Norris Hall, a classroom building a half-mile away on the other side of the 2,600-acre campus. At least 15 people were hurt in the second attack, some seriously. Many found themselves trapped after someone, apparently the shooter, chained and locked Norris Hall doors from the inside. Students jumped from windows, and students and faculty carried away some of the wounded without waiting for ambulances to arrive. Police commandos swarmed over the campus. A student used his cell-phone camera to record the sound of bullets echoing through a stone building. Inside Norris, the attack began with a thunderous sound from Room 206 - "what sounded like an enormous hammer," said Alec Calhoun, a 20-year-old junior who was in a solid mechanics lecture in a classroom next door. Screams followed an instant later, and the banging continued. When students realized the sounds were gunshots, Calhoun said, he started flipping over desks to make hiding places. Others dashed to the windows of the second-floor classroom, kicking out the screens and jumping from the ledge of Room 204, he said. "I must've been the eighth or ninth person who jumped, and I think I was the last," said Calhoun. He landed in a bush and ran. Calhoun said that the two students behind him were shot, but that he believed they survived. Just before he climbed out the window, Calhoun said, he turned to look at his professor, who had stayed behind, apparently to prevent the gunman from opening the door. The instructor was killed, Calhoun said. Erin Sheehan, who was in the German class next door to Calhoun's class, told the student newspaper, the Collegiate Times, that she was one of only four of about two dozen people in the class to walk out of the room. The rest were dead or wounded, she said. She said the gunman "was just a normal-looking kid, Asian, but he had on a Boy Scout-type outfit. He wore a tan button-up vest, and this black vest, maybe it was for ammo or something." The gunman first shot the professor in the head and then fired on the class, another student, Trey Perkins, told The Washington Post. The gunman was about 19 years old and had a "very serious but very calm look on his face," he said. "Everyone hit the floor at that moment," said Perkins, a second year student studying mechanical engineering. "And the shots seemed like it lasted forever." At an evening news conference, Police Chief Wendell Flinchum refused to dismiss the possibility that a co-conspirator or second shooter was involved. He said police had interviewed a male who was a "person of interest" in the dorm shooting and who knew one of the victims, but he declined to give details. Steger said authorities believed the shooting at the dorm was a domestic dispute and mistakenly thought the gunman had fled the campus. "We had no reason to suspect any other incident was going to occur," he said. Steger emphasized that the university closed off the dorm after the first attack and decided to rely on e-mail and other electronic means to spread the word, but said that with 11,000 people driving onto campus first thing in the morning, it was difficult to get the word out. He said that before the e-mail was sent, the university began telephoning resident advisers in the dorms and sent people to knock on doors. Students were warned to stay inside and away from the windows. "We can only make decisions based on the information you had at the time. You don't have hours to reflect on it," Steger said. The 9:26 e-mail had few details: "A shooting incident occurred at West Amber Johnston earlier this morning. Police are on the scene and are investigating." The message warned students to be cautious and contact police about anything suspicious.