The chief of Miami's police force has announced that he will resign, effective Jan. 1, after two and one-half years in the job. No official reason was given in Raul Martinez' letter of resignation, media reports said on Saturday.

However, his announcement came two days after a series in the Miami Herald that "documented dozens of questionable shootings involving Miami police officers" during the past decade. Martinez, the newspaper said, was chairman of the Firearms Review Board "that cleared at least 18 of those questionable shootings -- despite evidence that there were problems with the shootings or they might not have happened as officers claimed."

Martinez, who has been with the force for 28 years, was appointed to his post in 2000 by then City Manager Donald Warshaw and was the first Hispanic to hold the position.

Current City Manager Carlos Gimenez said he didn't intend to name a replacement before January.

The Herald said there were two apparent candidates to replace Martinez: former Miami Beach Police Chief Richard Barreto and Ignacio Vasquez, described as a high-ranking Miami-Dade officer.