SAN FRANCISCO, March 9 (UPI) -- San Francisco officials said the mess left by this year's massive Valentine's Day pillow fight has led them to take another look at the "flash mob" phenomenon.

The pillow fight, which marked its fourth year in February, involved an estimated 1,500 to 3,000 people at Justin Herman Plaza and left the city with thousands of dollars worth of damages and cleanup costs, the San Francisco Chronicle reported Monday.

Lisa Seitz Gruwell of the San Francisco Recreation and Park Department said organizers of the event must begin taking responsibility for the event, "otherwise we are going to have to find a way to shut it down."

However, officials said it is difficult to track down the organizers of the pillow fight since it is part of the flash mob fad, in which events are largely promoted through the Internet, text messaging and word of mouth. Flash mobs typically involve a large group of people gathering in a public place, engaging in unusual behavior and dispersing.

Mohammed Nuru, deputy director of the Department of Public Works, said the city had to dispatch 69 employees and an extra street sweeper truck to clean up after this year's pillow fight.

"It was quite a mess, much more than we have experienced in previous years," he said. "Everywhere was feathers."