BLACKSBURG, Va. - (AP) - A report of a possible gunman at VirginiaTech on Thursday set off the longest, most extensive lockdown andsearch on campus since the bloodbath four years ago that led theuniversity to overhaul its emergency procedures. No gunman was found, and the school gave the all-clear justbefore 3 p.m., about five hours after sirens began wailing andstudents and staff members started receiving warnings by phone,email and text message to lock themselves indoors. Alerts were alsoposted on the university's website and Twitter accounts. Maddie Potter, a 19-year-old from Virginia Beach, holed upinside a campus wood shop, where she had been working on a classproject. Staff members locked the doors and turned off the lights. "I was pretty anxious. We had family friends who were up herewhen the shooting took place in 2007, so it was kind of surreal,"she said. "I had my phone with me and I called both my parents." The emergency was triggered by three teens who were attending asummer program on campus and told police they saw a man walkingquickly across the grounds with what might have been a handguncovered by a cloth, authorities said. Police searched some 150 buildings on the square-mile campus andissued a composite sketch of a baby-faced man who was said to bewearing shorts and sandals, but they found no sign of him. Theycontinued to patrol the grounds as a precaution even after thelockdown was lifted. "We're in a new era. Obviously this campus experiencedsomething pretty terrible four years ago," said Virginia Techspokesman Larry Hincker. He added: "Regardless of what yourintuition and your experience as a public safety officer tells you,you are really forced to issue an alert." It was the first time that the entire campus was locked downsince the shooting rampage in 2007 that left 33 people dead, andthe second major test of Virginia Tech's improved emergency alertsystem, which was revamped to add the use of text messages andother means besides email of warning students. The system was also put to the test in 2008, when an explodingnail gun cartridge was mistaken for gunfire. But only one dorm waslocked down during that emergency, and it reopened two hours later. Earlier this year, federal authorities fined Virginia Tech$55,000 for waiting too long to notify staff and students after twostudents were shot to death at a dorm during the 2007 rampage. Anemail alert went out more than two hours later that day, about thetime student Seung-Hui Cho was chaining the doors to a classroombuilding where he killed 30 more people and himself. It was thedeadliest school shooting in U.S. history. This time, several thousand students and the school's 6,500employees were on campus for summer classes, officials said. Mostof Virginia Tech's 30,000 students are on summer break and won'treturn until the fall semester, which begins Aug. 22.