The choice of Mr Zedillo, like Colosio an economist and close friend of President Carlos Salinas de Gortari, was seen as a signal of the continuity of Mr Salinas's neo-liberal economic reforms. It left Mr Salinas open to further criticism, however, that his focus on economic reform has left him out of touch with political and social reality.
The announcement was almost overshadowed by growing indications that a disgruntled PRI faction may have been behind last week's killing of Colosio, 44, in Tijuana. Tranquilino Sanchez Venegas, 57, a retired policeman hired as part of Colosio's security team by a local PRI official on the day of the assassination, was arrested on Monday. He was taken from Tijuana to the same maximum-security jail near Mexico City as Mario Aburto Martinez, the 23-year- old accused of shooting Colosio. Mr Sanchez's family described him as a 'dyed-in- the-wool priista'.
Mexico's attorney-general's office said that Mr Sanchez's arrest was the result of a dramatic amateur video film that shows a gunman place a revolver virtually against the candidate's head and fire, all within a split-second. The video suggested that Mr Sanchez, seen behind the gunman, had cleared the way for him to get close to Colosio.
News of the ex-policeman's arrest and his reported close friendship with Rodolfo Rivapalacio, local PRI 'co-ordinator' in Tijuana, added fuel to conspiracy theories, most of them blaming elements within the party that has ruled Mexico for 65 years. Another former policeman, Vicente Mayoral, 47, questioned for several days, has since been released, but Mr Aburto's father was quoted as saying his son had spent a lot of time with Mr Mayoral in the weeks before the assassination.
Mr Sanchez's family denied he had been involved, but said Mr Rivapalacio had picked him up on the morning of Colosio's visit. Mr Sanchez is seen in the video wearing a dark baseball cap and a white armband with the candidate's name, marking him as part of the security team recruited locally by Mr Rivapalacio.
The attorney-general's office issued a written denial of media reports that linked Mr Sanchez with General Domiro Garcia Reyes, the officer in charge of the presidential candidate's security.
Justice officials said the team hired locally by the PRI for the day were mainly for crowd control and were not working in conjunction with Colosio's own travelling team of armed guards.
Mr Zedillo, a Yale graduate and father of five who once did a year-long course at Bradford University, is a former minister of both budget and planning and education.