An image that NBC News say they received from Cho Seung-Hui, the shooter in the Virginia Tech shootings, is seen as it is aired on the NBC Nightly News, April 18, 2007.(Xinhua Photo)

The gunman in Monday's shootings at Virginia Tech sent material to NBC News between the first and second shooting incidents that together killed 33 people, including the gunman himself, police said Wednesday.

"Earlier today, NBC News in New York received correspondence that we believe to have been from Cho Seung-Hui, the gunman responsible for the fatal shootings in Norris Hall," Steve Flaherty, superintendent of Virginia State Police, said at a news conference at Virginia Tech, located in Blacksburg, in the southwest of Virginia.

NBC immediately notified authorities after it received the material Wednesday, he said.

The material might be "a very new, critical component" of the investigation, he said.

"We're in the process right now of attempting to analyze and evaluate its worth," he said.

The material in the package was said to contain digital images of the gunman holding weapons and reading a manifesto declaring that he wanted to get even with rich people.

A total of 33 people, including the gunman, were killed in Virginia Tech Monday morning, in what had become the deadliest campus shooting incident in U.S. history, and police had identified the gunman as 23-year-old South Korean native Cho Seung-Hui, a senor majoring in English at the state university.

Editor: Yan