The masked man known as Subcomandante Marcos, spokesman for the Zapatista National Liberation Army (EZLN), accused the army of dropping incendiary bombs in Chiapas 'with strange gases reminiscent of chemical warfare' and of doubling its troop strength. As a result, the Zapatistas had shelved a peace plan reached with government and mediators from the Catholic Church in February, the statement said.
The army denied breaking the ceasefire. It admitted stepping up its activity in Chiapas but described the action as 'administrative movements'.
The increased tension in Chiapas, which borders Guatemala, swung attention from Tijauna on the US border, where Colosio, presidential candidate of the ruling Institutional Revolutionary Party (PRI), was shot while campaigning last Wednesday. But it did not stop a growing debate over who killed Colosio and why.
Few Mexicans suggest the Zapatistas had anything to do with the assassination. As reports of the killing increasingly fail to fit, conspiracy theories have swept the country. Many Mexicans blame the CIA, or the former US presidential candidate, Ross Perot. They say Mr Perot is desperate to stop the North American Free Trade Agreement (Nafta) between the US, Mexico and Canada, from going ahead. Most, however, believe the killing was a Mexican plot linked with rifts within the PRI itself.
The killing, although more reminiscent of the shooting of Robert Kennedy in 1968, is threatening to become a Mexican version of the aftermath of President John F Kennedy's killing in 1963. Although a 23-year-old Mexican, Mario Aburto Martinez, was seized with the alleged murder weapon, a .38 revolver, at the scene, a surgeon in Tijuana suggested a second weapon, possibly of .22 calibre, was involved. Colosio was shot twice, once in the head and once in the abdomen.
Justice officials in Tijuana have yet to make clear whether two other men detained with Mr Martinez, one of them a policemen, were involved. Mr Martinez, now in jail near Mexico City, has been quoted as saying he did not act alone and that he had had contact with unnamed armed groups in the past. According to media reports here, he told his interrogators he intended only to wound the presidential candidate.
But a mysterious photograph, taken from a video, shows a revolver close to Colosio's ear. The daily Excelsior published the picture but did not say where the video originated.
Despite the presence of many television crews and photographers, no film or picture of the killing or Colosio afterwards has emerged. 'Most newsmen would dwell on the victim. But where are the pictures? Nobody has seen Colosio,' said one Mexican cameraman.
Most Mexicans, showing little faith in a justice system largely controlled by the ruling party, say the truth may never be known. President Carlos Salinas de Gortari has appointed a Supreme Court justice, Miguel Montes Garcia, as special investigator. At a noisy meeting of Congress's permanent commission on Saturday night, however, the opposition Party of the Democratic Revolution (PRD), the PRI's main rivals in the elections scheduled for 21 August, questioned Mr Montes's objectivity.
The PRD leader, Porfirio Munoz Ledo, said Mr Montes was 'clearly linked, personally and politically, with the powers that be, against whom, founded or unfounded, there have been suspicions with regard to this crime'. Mr Munoz Ledo believes Colosio was killed by 'members of the oligarchy' who opposed his promises of greater democracy.