Tabasco state Attorney General Rafael Gonzalez said gunmen from the Zeta gang killed the mother, two siblings and an aunt of marine Melquisedet Angulo. He said four Zeta associates believed to have indirect roles in the attack had been detained, but the killers remained at large.
The slaying of Angulo's relatives early Tuesday just hours after his memorial service was widely viewed as a chilling warning from the Beltran Layva cartel that the families of soldiers and police could now suffer for the government's campaign against drug traffickers.
President Felipe Calderon called the attack on the marine's family "a cowardly act" and vowed to press forward with his war on the cartels involving more than 45,000 Mexican troops.
Angulo was the only marine who died in a Dec. 16 raid that set off a two-hour firefight thatand six of his bodyguards.
The Zetas, former military elite-turned-hit men, have allied with the Beltran Leyva cartel in recent years.
The detained suspects - three men and one woman - either served as lookouts for the Zeta hit men involved in the actual slaying or transported money to pay the gunmen, who are still at large, Gonzalez said.
"The motive was an agreement this group made as a result of the events of Dec. 16," he said at a news conference.
The four were paraded before reporters, their names written on white sheets of paper taped to the bullet-proof vests they were wearing.
Mexico's drug gangs frequently employ networks of lookouts and informants to perform logistics. Police have also been found to be on cartel payrolls, and Gonzalez later told local news media that some officers may have allowed - or helped - the assailants escape after Tuesday's attack on the family home in the Gulf coast state of Tabasco.
Cartels sometimes stage retaliatory hits on military or law enforcement after the arrests or killings of top traffickers. In Tabasco, they had previously made two such attacks on police officers and their families.
Assailants tossed hand grenades at government offices in the northern state of Sonora late Tuesday. No one was injured, but state officials there said they were on alert for possible reprisals for Beltran Leyva's death.
Mexico's National Human Rights Commission on Wednesday questioned the policy of releasing the names of soldiers and police who have died fighting drug cartels. The government commission suggested officials respect their right to privacy.
Dozens of troops and police guarded the funeral of Angulo's four relatives Wednesday, blocking public access to the cemetery in Tabasco.
Even before the marine's family was attacked, federal officials had warned that the death of Beltran Leyva, known as the "boss of bosses," could provoke a violent backlash from his gang.
The cartel's violent rage may have been further stoked this time by graphic photos published in local and international news media of Beltran Leyva's bullet-riddled body following the gun battle with marines at an apartment in Cuernavaca, south of Mexico City.
The photos showed Beltran Leyva with his pants pulled down to his knees and blood-soaked money strewn over his corpse.
Four employees of the state medical examiner's office are under investigation for manipulating a crime scene, according to the attorney general's office in the central state of Morelos, where Beltran Leyva was killed.
The office said Wednesday that the four could be fired, banned from working for the government and fined for misconduct.
Beltran Leyva was among the most-wanted drug lords in Mexico and the United States and was the biggest trafficker taken down by Calderon's administration so far. U.S. Drug Enforcement Administration officials blame his cartel for much of the bloodshed across Mexico.
More than 15,000 people have been killed by drug violence since Calderon's military-led crackdown against organized crime began in late 2006.
On Wednesday, eight bodies were found in three clandestine graves south of Chilpancingo, capital of the Pacific coast state of Guerrero. The state has been a major battleground for warring cartels, including the Beltran Leyva gang.
Police, who received an anonymous phone call about the location of the graves, said the eight unidentified victims had been shot dead up to a month ago.