From the Detroit News:

According to a criminal complaint, (Leon Desmond Barrett, 37,) sent about 14 threatening letters to the NFL Hall of Fame in Canton, Ohio and NFL Commissioner Roger Goodell between 2006 and April. 

Some of the letters referred to an alleged dispute between Barrett and Bettis and his family. “We do not want to have a gunfight with them but if we have to, we will,” said one of the hand-written letters.

“I guess it (sic) going to be another Virginia Tech. massacre all over again in Detroit.”
A letter dated Aug. 18, 2007, said: “I’m getting ready to start killing some more people. Beyonce, Jay-Z, Jerome Bettis and Tune-up Man are the first 4 people I’m getting ready to kill.”

Similar letters were sent in 2009 to former Miami Dolphins quarterback Dan Marino, care of the Hall of Fame, and former University of Notre Dame football coach Lou Holtz, the complaint said.

Most of the letters bore Barrett’s signature and home address, the complaint alleges.