Joseph Morales Serrano was taken into custody at his home in San Juan on a charge of cyberbullying for allegedly posting the threatening message on May 6, according to Special Agent Moises Quinones, a spokesman for the FBI in the U.S. island territory.
The suspect was expected to make an initial appearance before a federal judge Monday on the charge, which carries a maximum sentence of five years in prison. Morales has not entered a plea and did not yet have a lawyer, Quinones said.
Morales has acknowledged he posted a Twitter message in Spanish that included the words ‘‘watch out during the demonstration, it can end like in Boston,’’ the FBI said in a statement.
The message was directed at Pedro Julio Serrano, a well-known activist in Puerto Rico who was planning to attend an upcoming demonstration in support of gay rights legislation in the territory.
Serrano said he does not know Morales but believes he is responsible for previous threatening messages. He had asked police in Puerto Rico and New York, where he lives part of the time, to investigate first, then went to the FBI after the message was posted with an apparent reference to the April 15 bombing at the Boston Marathon.
‘‘He looked at me as a target because of my activism. It was not personal,’’ he said.
He said he has not heard the results of any investigation by police in Puerto Rico or New York.
From the Associated Press