Colosio's Death A Conspiracy After All, Say Probers -- Even Though Mexican Investigators Have Again Decided That Presidential Candidate Luis Donaldo Colosio's Assassination Was A Conspiracy, Observers Are Cautioning Not To Read Too Much Into The Assertion. Prosecutors Either Had To Charge A Second Man With Murder Or Drop The Charges.

MEXICO CITY - The murder of presidential candidate Luis Donaldo Colosio was a conspiracy after all, the fourth team of prosecutors investigating the assassination appears to have determined.

Relying on basically the same evidence that previous special prosecutors used to deduce that triggerman Mario Aburto Martinez acted alone last March 23, the new investigators have reached a radically different conclusion, say sources close to the case.

Special prosecutor Pedro Chapa is said to be proceeding with the theory originally advanced by federal police immediately after Colosio was shot following a campaign rally in a working-class neighborhood of Tijuana: The candidate died as a result of a conspiracy.

Chapa earlier this week brought formal charges of conspiracy to murder against Tranquilino Sanchez, one of three men who have languished in jail since April, when they were arrested as Aburto's co-conspirators, the attorney general's office confirmed.

Aburto was sentenced last year to 42 years in prison, but there had been no previous indication of progress in the trials of the three other men. Judge Alejandro Sosa refused defense lawyers' requests that charges be dismissed against Vicente and Rodolfo Mayoral, a father and son accused in the alleged conspiracy.

In addition, investigators are now reported to be considering the possibility that a second gun was fired at Colosio.

Still, criminal lawyers cautioned against reading too much into the new special prosecutor's actions. Mexican law limits the time prosecutors have to present conclusions and formal charges, which come at the end of a trial here. Facing a deadline, Chapa had to present conclusions accusing Sanchez or drop the charges.

Sources close to the investigation said Chapa may have felt pressured to push for a conviction because the lone-gunman theory has met with widespread public skepticism.

When the first special prosecutor, Miguel Montes, concluded that Aburto acted alone, Colosio's late widow hired her own attorney, noted litigator Juan Velazquez.

By asking for a conviction based on basically the same evidence used to bind Sanchez over for trial last spring, Chapa places the onus on Sosa. After reading the defense response, which must be presented in 30 days, Sosa will consider the evidence and reach a verdict. Mexico does not have jury trials.

Sosa has already released for lack of evidence one of the men originally arrested by federal police. Evidence against Sanchez and the Mayorals, all members of the crowd-control team assigned to the rally, consists mainly of videotapes showing them pushing and shoving as Aburto makes his way through the crowd.

In addition, Aburto's friend Graciela Gonzalez told police that she saw him talking with Sanchez less than two weeks before the murder. Other witnesses have said they saw the two men talking before the rally.