What are "Anonymous Letters"?

Tom and Huck first plant a letter reading, "Beware. Trouble is brewing. Keep a sharp lookout." The next night the boys tack up a letter containing a skull and crossbones, which they follow with a picture of a coffin.

Tom plans a final coup by drafting a longer letter. Pretending to be a member of a gang of robbers who are planning to steal Jim from the family, he warns them that the gang will be coming late at night from the north to get Jim. The family is terribly frightened and does not know what to do.