WASHINGTON – Police and prosecutors are worried that a Web site claiming to identify more than 4,000 informants and undercover agents will cripple investigations and hang targets on witnesses.

The Web site, WhosaRat.com, first caught the attention of authorities after a Massachusetts man put it online and named a few dozen people as turncoats in 2004. Since then, it has grown into a clearinghouse for mug shots, court papers and rumors.

Federal prosecutors say the site was set up to encourage violence, and federal judges around the country were recently warned that witnesses in their courtrooms may be profiled online.

“My concern is making sure cooperators are adequately protected from retaliation,” said Chief Judge Thomas Hogan, who alerted other judges in Washington’s federal courthouse. He said he learned about the site from a federal judge in Maine.

The Web site is the latest unabashedly public effort to identify witnesses or discourage helping police. “Stop Snitching” T-shirts have been sold in cities around the country and popular hip-hop lyrics disparage or threaten people who help police.

In 2004, NBA star Carmelo Anthony appeared in an underground Baltimore DVD that warned people they could be killed for cooperating with police. Anthony has said he was not aware of the DVD’s message.

Such threats hinder criminal investigations, said Ronald Teachman, police chief in New Bedford, Mass., where murder cases have been stymied by witness silence.

“Every shooting we have to treat like homicide. The victim’s alive but he’s not cooperative,” Teachman said. “These kids have the idea that the worst offense they can commit is to cooperate with the police.”

Sean Bucci, a former Boston-area disc jockey, set up WhosaRat.com after federal prosecutors charged him with selling marijuana in bulk from his house. Bucci is under house arrest awaiting trial and could not be reached, but a WhosaRat spokesman identifying himself as Anthony Capone said the site is a resource for criminal defendants and does not condone violence.

“If people got hurt or killed, it’s kind of on them. They knew the dangers of becoming an informant,” Capone said. “We’d feel bad, don’t get me wrong, but things happen to people. If they decide to become an informant, with or without the Web site, that’s a possibility.”