BLAKSBURG, Va. (ABC News) - At least 29 people are dead in what may be the biggest mass killing on a college campus in American history — and the death toll may rise.

Police at Virginia Tech, in Blaksburg, Va., said that the shootings happened at a dormitory and a classroom on opposite sides of the university campus.

Law enforcement sources tell ABC News the shooting may have been set off by an off-campus incident. Details were unclear.

Virginia Tech Police Chief Wendell Flinchum said that one person was killed in the first shooting, which occurred just after 7 a.m. at West Ambler Johnston Hall, a large dormitory. Flinchum said that at least 20 more people were killed at Norris Hall, an academic building.

The gunman, whose identity has not been released, is among the dead. Flinchum wouldn't say whether the shooter had killed himself.

ABC News has confirmed that there were two separate bomb threats last week at Virginia Tech that targeted engineering buildings. The first was directed at Torgersen Hall, while the second was directed at multiple engineering buildings. Students and staff were evacuated, and the university had offered a $5,000 reward for information into the threats.

Two local hospitals have reported treating 21 people, though it is unclear how many of those may have died.

University president Charles Steger said that police have not officially tied together the two shootings. Steger described today's events as an incident of "tragic proportions" and said, "the university is shocked and horrified that this would befall us."

The campus was closed Monday and today’s classes have been canceled. Families seeking to reunite with their children have been directed by the university to the Inn at Virginia Tech.

Virginia Tech, located in western Virginia near the borders of West Virginia and Tennessee, has more than 25,000 full time students. Its campus, which spreads out over 2,600 acres, has more than 100 buildings.

S. Daniel Carter, senior vice president for Security On Campus, Inc. who has been studying campus crime for 15 years, said he watched the news at Virginia Tech unfold in "utter shock." Carter is based a couple of hours' drive away, in Knoxville, Tenn.

"Nothing like this has happened before," he said, adding that the average number of killings on all American campuses combined has hovered around 20 since the Department of Education formally began collecting data in the early 1990s.

While information had not been released about the gunman, Carter said he suspected that whoever the shooter was suffered from some deep psychological problem and was likely connected to the university campus in some way. "In the past, in similar cases, it's usually been a psychological issue and not just a security issue," Carter said. "One of the people who was killed was an older individual, maybe a faculty member. That could be a likely underlying factor in this case - someone who has failed." "It is difficult to comprehend senseless violence on this scale," said Virginia's Governor Timothy M. Kaine in a statement. "Our prayers are with the families and friends of these victims, and members of the extended Virginia Tech community."