A Waldorf couple who rented a neighborhood community center for a family gathering on Thanksgiving day found a message of hate pushed under the center's door. Yesterday, authorities said they are investigating its origin.

Crawling across the white photocopied page of small type, authorities said, were threats of arson and retribution against blacks and other minority groups. The letter also contained nonspecific threats against local law officers and courthouse security guards, the Charles County sheriff's office said.

The letter left at the Lancaster Neighborhood Center was one of 30 similar letters found on community mailboxes Thursday throughout the Lancaster neighborhood and on nearby Amber Leaf Place, all in Waldorf's St. Charles community, the sheriff's office said. Authorities did not release the text of the letters yesterday, citing the continuing investigation.

Authorities said they do not believe that any specific resident was the intended target of the letters, in part because the letters did not name individuals or families.

"We're taking the threat seriously and doing what we can to investigate it," said Craig J. Renner, spokesman for the Charles County sheriff's office. Renner said additional security measures were taken yesterday at the Charles County Courthouse in La Plata in response to the letters' threats. In addition, sheriff's patrols were increased in the Lancaster neighborhood.

Renner also said that Charles County sheriff's investigators have conferred with police at the University of Maryland at College Park, where student leaders, the Afro-American studies department and the Black Student Union received mailed hate messages this month. Renner said sheriff's investigators do not believe that the two incidents are related.

Authorities were contacted Thursday both by the couple at the Lancaster Community Center and a female resident of Grouse Place who found several of the letters on top of a group mailbox. All three told authorities they did not want to publicly discuss what they found.

Sheriff's investigators canvassed the neighborhood and eventually determined that photocopies of two slightly differing versions of the hate letter were also left on community mailboxes on Falcon Place, Amber Leaf Place and New Forest Court, Renner said. The Lancaster neighborhood consists of 1,300 houses, town homes and apartments in St. Charles, the planned community of 30,000 residents at the heart of Waldorf.

Residents expressed surprise at news of the threatening letters yesterday.

"The way I look at it, it's a copycat of what's going on in College Park," said Linda Ivko, the president of the Lancaster Neighborhood Association. "We don't have any racial problems here that I know of. . . . I don't see it as a true issue. I see it as somebody trying to get attention or copying what happened in College Park."