A journalist was killed this week as he left a holiday party in the Caribbean resort town of Tulum, human rights officials said Friday, bringing to 12 the number of reporters killed this year in the country. The journalist, Alberto Velázquez, who worked for the newspaper Expresiones de Tulum, was killed Tuesday. According to the Committee to Protect Journalists, Mr. Velázquez had written articles critical of local officials, and his paper had received threats. In a statement, the group quoted colleagues as saying Mr. Velázquez was shot by a gunman on a motorcycle, and they believed it was related to his reporting.