Colosio Guard Charged as ‘Co-Participant’ : Inquiry: Special prosecutor heads for Tijuana as authorities try to quell rumors.

As a special prosecutor was about to begin trying to unravel the increasingly mysterious case surrounding the assassination of Mexico’s ruling party presidential candidate, a security guard was formally charged Tuesday in the capital with being a “co-participant” in the killing.

Authorities also seemed to spend much time seeking to dispel the myriad rumors sweeping the nation about the assassination of Luis Donaldo Colosio, the candidate who was gunned down here a week ago today but had been widely expected to be Mexico’s next president.

Among the developments:

* U.S. officials downplayed allegations by Mexican investigators that the chief suspect in the case, Mario Aburto Martinez, had links with gangs or other unspecified groups in Los Angeles.

* Mexican authorities insisted that they will run a meticulous, impartial investigation. They sought to refute the widespread suggestion of a conspiracy involving Mexican officials in Colosio’s killing, which would taint the ability of government investigators to fairly probe the case.

* Mexican officials also have insisted that Colosio was slain by two shots--not three, as some reports suggested--and that the gun they have in evidence is, indeed, the weapon used in the assassination. New questions have arisen about the weapon because of its appearance in a widely broadcast, brutal videotape of Colosio’s wounding at point-blank range.

Investigators Tuesday intensified their focus on the security team assembled by the Institutional Revolutionary Party, or PRI, for Colosio’s fatal campaign stop here.

At least two security guards in the PRI-arranged force, both former policemen, may have had previous contacts with the accused assassin; both former officers were at the assassination site in a poor Tijuana barrio, with one reportedly arriving in the neighborhood with Aburto on the day of the killing.

Tranquilino Sanchez Venegas--a guard who authorities say was seen with Aburto in a Tijuana park a week before the killing--was flown to a high-security prison near Mexico City and charged as a “co-participant.” Authorities said he may have cleared the crowd to let Aburto near Colosio.

Relatives of Sanchez said Tuesday that the former policeman is innocent and never knew Aburto, the accused assassin. “He’s incapable of doing this,” Maria Ramirez Sanchez said of her husband.

The other guard, Vicente Mayoral Valenzuela, was initially arrested in the melee following the shooting but later released.

Meanwhile, U.S. officials downplayed a suggestion by the Mexican attorney general’s office that Aburto had links with unspecified “groups” and “gangs” in the Los Angeles area, where the suspect had lived in recent years. “That’s purely speculation at this time,” said John Hoos, the FBI spokesman in Los Angeles.

Mexican officials provided little substantiation for their claim of Aburto’s Los Angeles links other than noting that he had a nickname, “Eagle Knight,” after an Aztec warrior symbol, which is a popular emblem of Mexican nationalism.

A police gang expert told The Times the symbol has no special significance for Los Angeles gangs.

Miguel Montes Garcia, a former Supreme Court justice named as special prosecutor in the case, was scheduled to arrive in Tijuana late Tuesday to oversee the high-profile inquiry.

Rotella reported from Tijuana and McDonnell from Los Angeles.