Investigators think the carbon monoxide fumes came from a faulty gas water heater, reports CBS News correspondent Bob Orr.
The leak was discovered late 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. Fall semester classes begin Monday.
The two students critically sickened were at the University of Virginia Hospital in Charlottesville. Their three roommates, all 19-year-old sophomores, were in stable condition at Duke University Medical Center in Durham, N.C.
Eighteen other people were treated at hospitals and released Sunday.
At a weekend ceremony there were 32 bells, 32 families, and 32 limestone markers honoring those students and faculty killed in the April rampage, adds Orr.
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 as he munched on a granola bar on his way to class.
Tech enrolled a record freshman class of 5,200 for the fall, but university spokesman Mark Owczarski said officials won't know for a couple of weeks exactly how many of the 26,000 students returned this fall. Of those who withdrew before classes started, he said, only two reported they were doing so because of the shootings.
"There doesn't appear anything out of the ordinary," he said. "It's normal numbers."
One change is that no classes will be held in Norris Hall, where most of the killings took place. On Monday, flowers lay at entrance to the building, now being used exclusively for engineering laboratories and offices.
The building was closed entirely for nearly two months, delaying research for students in the engineering science and mechanics department. Graduation was delayed a semester for two students, department chairman Ishwar Puri said Sunday.
"Coming back to Norris Hall is not as simple as we expected," he said. "The challenge, really, is the emotional state of the group as a whole."