Ulises Grajales Niño, who had been the candidate of Mexico's Institutional Revolutionary Party, or PRI, candidate for mayor of the southern city of Villaflores, was arrested in connection with the murder of a member of a rival party, the Chiapas state Attorney General's Office said.

Grajales was arrested Tuesday after investigators confirmed that he was present when Edgar Hernandez Corzo, a National Action Party, or PAN, member, was murdered, the AG's office said.

A test indicated that Grajales did not fire a weapon, but his driver and bodyguard, Alfonso Lopez Moreno, tested positive, the AG's office said.

The two men were arrested on homicide charges, state prosecutors said.

Investigators are gathering evidence in the case and have statements from eyewitnesses who said Grajales took part in the killing of Hernandez Corzo, the 25-year-old nephew of congressional candidate Roger Decoss Corzo, early Monday, the AG's office said.

Eyewitnesses told investigators Grajales was riding in the white SUV from which the shots that killed Hernandez Corzo were fired, the AG's office said.

Hernandez Corzo was murdered near the El Sinaloense ranch on the Villaflores-Chanona highway as he returned with two other people from painting PAN campaign slogans in the area,

The other two occupants of the automobile told investigators that shots were fired at them as they headed to the ranch, the AG's office said.

Grajales and his bodyguard got out of their vehicle and threatened to kill Hernandez Corzo's companions in the presence of several other people, the AG's office said, citing eyewitnesses accounts.

The PRI politician took his bodyguard to a house he owns in Villaflores, where the suspect hid until his arrest, the AG's office said.

Grajales Niño's candidacy has been cancelled and he should cooperate with investigators to clear up the incident, the PRI's state chairman in Chiapas, Roberto Albores Gleason, said in a statement released Monday.

"Our role as a political institution is without a doubt to not incriminate anyone, but we will not ignore acts of violence. As a result, we are asking the authorities to move quickly to clear up the legal status of Ulises Grajales," Albores said.

The PAN National Committee condemned Hernandez Corzo's killing and called on officials to investigate the incident "since an act of this nature cannot go unpunished, and we call for the punishment of those responsible to be expedited."

"National Action regrets this murder and expresses its most deeply felt condolences to the family of Hernandez Corzo," the PAN National Committee said.

A candidate for a seat in the Guerrero state legislature, meanwhile, was gunned down Monday at his house.

Margarito Genchi Casiano, a member of the leftist Party of the Democratic Revolution, or PRD, was killed around 7:00 a.m., the Guerrero Attorney General's Office said.

The politician was murdered by several gunmen who went to his house in Llano Largo, a community on Guerrero's Costa Chica.

Investigators have not determined the motive for the murder, which comes as the campaign for the July 1 general elections enters the home stretch.

Mexico's presidential campaign ends on June 27, four days before some 80 million voters head to the polls to cast ballots for a new president, 628 legislators and thousands of other officials. EFE