MEXICO CITY -- In the most compelling indication yet of a political conspiracy in the slaying last year of Mexico's leading presidential candidate, federal agents arrested a 28-year-old former employee of the governing party yesterday as the second gunman in the shooting.
The charging of a second suspect with murder and a disclosure that evidence in the crime had been tampered with, marked a complete reversal from the previous government's claim that the candidate, Luis Donaldo Colosio, was the victim of a lone, deranged assassin.
Officials said that the head of an undercover security unit working for Mr. Colosio's campaign was also arrested yesterday on charges of having lied about evidence gathered at the crime scene.
The break in the case comes less than three months after President Ernesto Zedillo took office promising a credible investigation of the killing.
Mr. Zedillo managed Mr. Colosio's campaign and assumed his candidacy after Mr. Colosio was shot to death as he left a rally in the border city of Tijuana on March 23, 1994.
Government officials declined to say who they now believe might have been behind the slaying, which jolted the party that has governed Mexico for six decades.
The opposition-party lawyer whom Mr. Zedillo appointed as his attorney general, Antonio Lozano Gracia, vowed to pursue the investigation wherever it leads.
The suspect charged last year, Mario Aburto Martinez, was arrested at the murder scene and was sentenced to 45 years in prison last December.
Officials said that the arrest of Othon Cortes Vazquez, in Tijuana, was based on the testimony of three witnesses, videotape from the campaign rally, and new evidence gathered by the third special prosecutor to be assigned to the case, Pablo Chapa Bezanilla.
The government-owned newspaper El Nacional reported yesterday that Mr. Cortes, who appears in a series of photographs taken at Colosio's last campaign rally, assisted the army general who headed the candidate's official security detail in the hours before and after the killing.
Immediately after the killing, government officials described Aburto as a demented loner who had apparently acted on his own.
But days later, the first of three special prosecutors named to the case, Miguel Montes Garcia, announced that at least six other men had helped the gunman push through the crowd to reach the candidate. Four of them were arrested, and the other two were described as fugitives.
As his case began to crumble into a heap of contradictory testimony, mishandled evidence, and neglected leads, Mr. Montes reversed himself. He never dropped charges against three of the four men accused of aiding Aburto, but issued a long report asserting that Aburto was driven to kill by his own, crazed hatred for the governing party.
Then Mr. Montes resigned from the investigation.
Mr. Lozano, the attorney general, said his investigators had virtually started the investigation over again and had quickly concluded that Aburto could not have acted alone.