Mexico City - Investigators began interviewing more top officials in the Gulf Coast state of Veracruz on Tuesday about the recent killing of a photojournalist, an activist and three other women in the Mexican capital.Mexico City prosecutors said they have requested statements from the current and former Veracruz public security secretaries as well as the head of a state commission responsible for protecting journalists.The commission said in a separate statement that its chief met with investigators on Tuesday. There was no information on when the other two officials would testify, but prosecutors said the investigators would be in the state for several days.Veracruz Governor Javier Duarte has also been interviewed about the killings and denied involvement.Freedom of expressionRuben Espinosa was a photographer who fled the state to seek refuge in Mexico City after receiving threats. His friend and activist Nadia Vera was an outspoken critic of the Veracruz government.Both were found dead late last month along with the three other victims in an apartment in a middle-class Mexico City neighbourhood. Family members and groups defending freedom of expression have urged investigators to determine whether they may have been killed because of their work.Fourteen journalists have been killed in Veracruz since Duarte took office in 2010 and three more have disappeared.One suspect has been arrested in the Mexico City case, based on a fingerprint found in the apartment. Daniel Pacheco Gutierrez faces charges of murder, femicide and aggravated robbery.More than 500 people including prominent journalists, artists and writers have signed a letter to President Enrique Pena Nieto demanding swift justice in the case and a review of media protections in Mexico. Signers included Mexican actors Gael Garcia Bernal and Diego Luna, filmmakers Alfonso Cuaron and Guillermo del Toro, journalist Carl Bernstein, intellectual Noam Chomsky and novelist Salman Rushdie.On Tuesday, the government responded with a statement condemning attacks on reporters and human rights activists and saying it "shares the concern over attacks against the physical integrity of any journalist".