BLACKSBURG, Va. – A Virginia Tech campus still reeling from the deaths of 32 people at the hands of a student gunman last spring began its fall semester Monday amid another tragedy: a carbon monoxide leak at an off-campus apartment left five roommates hospitalized, two in critical condition.

The leak appeared to be from a faulty valve in a gas water heater in the apartment the students shared, Blacksburg Police Capt. Bruce Bradbery said.

It was discovered Sunday morning after a neighbor complained of fumes, just as Virginia Tech was preparing to dedicate a memorial to the 27 students and five faculty members killed April 16 by Seung-Hui Cho. Bruce Bradbery was at the dedication ceremony when he got the call.

“Enough’s enough,” he said. Last fall, an escaped fugitive on the loose near campus had forced the university to shut down on its first day of classes.

Two students from the apartment complex remained in critical condition Monday at the University of Virginia Hospital in Charlottesville, a spokeswoman said. Their three roommates, also 19-year-old sophomores, were in stable condition at Duke University Medical Center in Durham, N.C. Eighteen other people sickened by the carbon monoxide were treated Sunday and released, Blacksburg police said.

On campus Monday morning, the school routine was back as thousands of Virginia Tech students hustled off to their first classes of the semester.

Matt Rebholz of Pittsburgh said the shootings had brought students closer.

“It’s a lot more of a family atmosphere,” the sophomore electrical engineering student said.

One change on campus is Norris Hall, the former classroom building where Cho killed all but two of his victims. On Monday, flowers lay at entrance to the building, now used exclusively for engineering laboratories and offices.