SAN JUAN (Puerto Rico) (AP) - A man has been arrested in Puerto Rico for a Twitter post that threatened a gay activist and made a reference to the Boston Marathon bombing, the United States Federal Bureau of Investigation (FBI) said on Monday.

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 US island territory.

The suspect was expected to make an initial appearance before a federal judge on 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, Agent Quinones said.

Morales has acknowledged that 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 Mr 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.

Mr Serrano said he does not know Morales, but believes he is responsible for previous threatening messages. He had first asked the police in Puerto Rico and New York, where he lives part of the time, to investigate, then went to the FBI after the Twitter 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 had not heard about the results of any investigation by the police in Puerto Rico or New York.