BOULDER, Colo. -- Boulder County sheriff's deputies say vandals painted two swastikas on the truck of a controversial University of Colorado professor.

Sheriff's spokesman Lt. Phil West says the vandals struck sometime between late Tuesday and early Wednesday while the truck was parked in front of Ward Churchill's home.

The swastikas may be in reference to an essay he wrote after the Sept. 11, 2001 terrorist attacks -- in which he compared the people killed in the World Trade Center to Nazis.

Churchill has been highly criticized for the essay, and many critics have called for his firing.

On Wednesday, Churchill turned over copies of hate mail to sheriff's deputies that he says he's received.

He also says he's received numerous death threats since the uproar over his essay.

"The Anti-Defamation League condemns the painting of one of history's most hateful symbols, the swastika, on Professor Churchill's truck," said Mountain States Regional Director, Bruce H. DeBoskey. "Although ADL has previously expressed its outrage at Professor Churchill's callous comments comparing World Trade Center workers to 'little Eichmanns,' hateful symbols are an inappropriate response and should not be tolerated in our community. All Coloradans should join together in condemning Nazi symbols and analogies, no matter who is responsible for them, where they appear, or against whom they are directed."