A memo sent to players last week by the NFL's security department said all the threatening letters came from the same person, The Palm Beach Post reported Thursday. The letters were postmarked Cleveland; Youngstown, Ohio; and Erie, Pennsylvania.
"There have been 11 or 12 letters directed to prominent individuals throughout the United States over the past two years," special agent Robert Hawk of the FBI's Cleveland office told the Post on Wednesday. "These letters contained veiled threats. There were overtures of violence."
The NFL's memo said at least six letters have been sent directly to players, their teams and their cheerleaders. One letter was sent to a New Jersey amusement park where a player was scheduled to appear.
Threats of violence have escalated with each letter, the memo said.
Hawk said the letter-writer appears to have also sent similar mail to black public figures, including entertainers.
"We do not have any suspects," Hawk said. "But we are currently conducting lab analysis to find the source. If we find this person, we will refer him for prosecution, because obviously what has been done is illegal."
NFL spokesman Greg Aiello said, "I think the memo speaks for itself."