Durham, N.C. — Duke University police are investigating death threats against professors who questioned racial attitudes on campus a year ago, shortly after a black woman accused three white Duke lacrosse players of sexually assaulting her.

The so-called "Group of 88" professors endorsed an advertisement in The Duke Chronicle newspaper that quoted students who talked candidly about racism on campus. Some readers said the faculty members were convicting the lacrosse players before they had been tried.

Prosecutors recently dropped all charges against the three players, saying there was no credible evidence that any assault occurred.

The professors denied theywere prejudging the players, but the ad was enough to spark violent, racist attacks against them in some Internet blogs.

The attacks have continued even as the case against the players has ended, said the professors, who didn't want to be identified for fear of instigating more threats. They said they receive about a half dozen threatening e-mails a day from unidentified senders.

"The white race is watching. I hope to see you hanging from a tree," reads one e-mail.

The professors said they also have received bizarre packages in the mail that they think were prompted by their endorsement of the newspaper ad.

Still, they said they don't regret endorsing the ad.