2nd Suspect May Have Aided in Colosio Killing : Mexico: Federal police now believe guard cleared path for gunman and are holding him for questioning.

Amid increasing indications of a conspiracy, Mexican federal police said Monday that they now believe a security guard aided the gunman accused of assassinating presidential candidate Luis Donaldo Colosio last week.

Investigators were questioning Tranquilino Sanchez Venegas, a retired police officer hired to provide crowd control, whom photos and videos of the incident show near Colosio and accused gunman Mario Aburto Martinez seconds before the fatal shots were fired.

A girlfriend of Aburto has told police that Aburto met with Sanchez in a Tijuana park a week before the March 23 assassination, according to Mexican federal officials, who suspect Sanchez of clearing a path through the crowd for the gunman.

Although Aburto is considered the lone gunman, police are investigating “the probable participation of accomplices, co-participants or protectors of Aburto Martinez, as is the case of (Sanchez),” said Deputy Atty. Gen. Rene Gonzalez de la Vega in a statement. “His behavior and his attitudes near the candidate and Aburto Martinez lead us to presume that he participated directly with the accused killer.”

These revelations provide the strongest support yet for what journalists, politicians and other Mexicans have been asserting for days: that Colosio, the leading candidate for Mexico’s presidency, was the victim of a plot involving more than one person. And the focus of the probe appears to be the possible involvement of members of a security team provided by local officials of the ruling Institutional Revolutionary Party, known as the PRI.

Until now, Mexican authorities had said that Aburto, 23, acted alone when he emerged from a crowd and shot Colosio twice at point-blank range after the candidate finished a speech.

But security guard Sanchez was detained for questioning this weekend because his actions raised investigators’ suspicions, U.S. and Mexican sources said. Videotapes and photographs of the assassination, which took place in the sloping dirt plaza of a shantytown, show Sanchez--a tall man wearing a baseball cap--in the crowd just behind Colosio. On the right, the gunman can be seen approaching and raising the gun to the candidate’s head as Colosio shakes hands with well-wishers.

Police suspect Sanchez aided Aburto by leading him through the crowd, according to sources in both nations. “They are reviewing the tapes to see if he’s opening a path for Aburto to get closer,” one official said.

Moreover, the girlfriend’s statement to authorities appears to establish a prior connection between Sanchez and Aburto, who in his confession said he practiced for the shooting with a .38-caliber revolver. He also hinted that others were involved, according to witnesses to the interrogation last week in Tijuana.

“The supposed girlfriend of Mario Aburto Martinez said that he and Tranquilino Sanchez Venegas had a meeting a week before in Tijuana,” said Hugo Morales, press secretary in the attorney general’s office. The meeting was in a public park, Morales said.

Sanchez is a retired officer of the commercial police, a municipal force that provides security for businesses, federal authorities said. He is described as being in his 50s.

Sanchez was hired on the morning of the assassination by Rodolfo Riva Palacio Tinajero, a leader of the PRI, for crowd control in Tijuana’s impoverished Lomas Taurinas neighborhood, authorities said.

In a news release Monday, the attorney general’s office emphasized that Sanchez had no connection to the commanding officer of Colosio’s personal security force--members of the Mexican equivalent of the Secret Service, who traveled with the candidate. Instead, Sanchez was one of about 45 local guards provided for crowd control, Mexican officials say.

Sanchez’s name, however, does not appear on a typewritten list of guards, American and Mexican sources said. His name was handwritten onto the list, apparently a late addition that has attracted the attention of investigators.

The team of Tijuana-based security guards included former city and state police officers who belong to the PRI and were assembled by officials of the local party branch, PRI officials said in interviews Monday.

The guards included Vicente Mayoral Valenzuela, 47, a former deputy chief homicide investigator of the state judicial police. He was arrested in the melee immediately after the shooting but was later released after federal authorities said he was merely a witness who helped subdue the gunman.

In a new development involving Mayoral, the father of the accused assassin told The Times on Monday that Mayoral had befriended Aburto months before the shooting. Mayoral is thus the second member of the Colosio security entourage provided by the PRI to be linked to Aburto before the assassination.

According to the father, Mayoral sought out Aburto late last year and bragged to the young man about his trips overseas to Russia, among other destinations, and also of his former friendship with Lazaro Cardenas, the revered former president of Mexico, who died in 1970. Mayoral maintained the friendship in recent months, the father said, although Aburto didn’t like him.

“I think Mayoral is the key to this,” said the father, Ruben Aburto, 47, who resides in San Pedro, Los Angeles’ harbor district, along with two of the suspect’s brothers.

Family members assert that Mexican officials are making Aburto the fall guy for a larger conspiracy to conceal the involvement of the government. Although they have no proof, relatives maintain that Aburto may have been forced to go to the scene, perhaps after alleged conspirators threatened to kill other family members in Tijuana.

Times staff writers Patrick J. McDonnell in San Pedro and Juanita Darling in Mexico City contributed to this report.

* ANOTHER VIEW: Suspect Mario Aburto Martinez is hard-working, intelligent, friends and family say. B1