SPRINGFIELD (Virginia), April 16: As many as 33 people were killed and 20 injured at Virginia Tech University on Monday in the biggest mass killing on a college campus in American history.

Police confirmed that most of the victims were students and that the gunman was also shot to death. But they are refusing to say what caused the shootings and who was behind it.

An earlier posting on the university’s website said the police had one shooter in custody and, as part of routine procedure, they continued to look for a second shooter.“There were two shootings on campus. In each case, there were fatalities,” said Virginia Tech President Charles Steger. “The university was struck today with a tragedy of monumental proportions,” Mr Steger remarked.

Virginia Polytechnic Institute and State University, better known as Virginia Tech, is one of the few universities in the US which also provide a full-time military training to cadets.

In a message posted on the university’s website, Mr Steger said the campus police received a 911 emergency call at 7.15am on Monday morning, reporting multiple shootings in West Amber Johnston Hall.

While in the process of investigating, about two hours later the university received reports of a shooting in its Norris Hall. In both dormitories, callers reported several victims. The injured have been transported to various hospitals in the immediate area in the region to receive emergency treatment.

Police official Wendell Flinchum told reporters that some of the victims were shot in the classroom. Mr Steger said the university was currently in the process of notifying families of victims.

“Crime scenes are being investigated by the FBI, University Police, and State Police. We continue to work to identify the victims impacted by this tragedy,” he added.

Engineering student Josh Wargo told ABC News he heard “almost 40 or 50 shots.” 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, a classroom and laboratory building, 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.

While information had not been released about the gunman, S. Daniel Carter, senior vice president for campus security who has been studying campus crime for 15 years, told reporters he suspected that whoever the shooter was suffered from some deep psychological problem.