NEW YORK - A student's shaky cell phone video with the sound of multiple gunshots provided the most riveting impression Monday as television networks rushed to cover the sickening massacre at Virginia Tech.

Blacksburg police officers run from Norris Hall on the Virginia Tech campus in Blacksburg, Va., Monday, April 16, 2007. [AP]

What first appeared to be a single shooting death unfolded into the worst gun massacre in the nation's history.

"On this day, a tranquil college campus became a killing field," ABC's Charles Gibson said at the outset of "World News."

A video shot by student Jamal Albarghouti appears unremarkable to the eye, with police officers at the end of a parking lot standing, guns drawn, outside a campus building. But the sound of sporadic pops captured what was going on.

Albarghouti told CNN that he at first believed he was happening upon the investigation of a bomb threat. There had been several on campus in recent weeks, he said.

"After a minute when I reached the area where I took the video from, I saw cops with guns and they were asking everyone to lay down or leave really quickly," he said. "I knew it wasn't another bomb threat. I knew it was something way more serious."

Cable news networks covered the story continuously Monday afternoon, and Albarghouti's footage topped each of the network evening newscasts. Broadcast networks also broke into regular programming several times, including when President Bush made a statement and Virginia Tech authorities held news conferences.

Anchors Katie Couric of CBS News and Brian Williams of NBC rushed to Blacksburg, Va., to originate their evening newscasts there. Wary of travel troubles due to the Atlantic nor'easter, ABC's Gibson held back and planned to head south after the news.

Williams stood in a stiff wind at the campus in a black overcoat, saying Americans will long remember what they were doing when they heard about the massacre.

"This is easily the saddest place in our nation tonight because this place will now be known forever as the scene of the largest mass shooting in U.S. history," he said.

CBS expanded its evening news to an hour, NBC planned a "Dateline NBC" special on the shootings in prime-time and ABC was expanding "Nightline." CNN's "Larry King Live" put off its celebration Monday of King's 50th year in broadcasting to cover the story.

"This is a story that has really shocked the nation," said CNN's Wolf Blitzer. He stood before a video wall that displayed several still pictures of the aftermath.

Fox News Channel, at the bottom of its screen, flashed a timeline of the shootings.

ABC's Web site helped to gather eyewitness accounts and served as a message board to help Virginia Tech students and parents keep track of their loved ones. The Web site posted links to social sites like Facebook where students were trying to watch out for one another, said Michael Clemente, ABC News senior executive producer for digital media.

"We can tell a story with this kind of depth and complexity faster than we have ever been able to," he said. "I don't think we could have had this kind of info in 12 or even 24 hours, before."