The abduction of one of jailed kingpin Joaquín "El Chapo" Guzmán's sons has triggered fears of a violent backlash — with reports suggesting his more important older brother might have been kidnapped as well.

The attack took place in the early hours of Monday morning in an upscale restaurant called La Leche in the western beach resort city of Puerto Vallarta.

According to the authorities, seven gunmen arrived at the restaurant around 1am and left it soon after that with six men in their power, including 29-year-old Alfredo Guzmán.

Law enforcement sources cited in the newspaper El Universal on Wednesday, said Iván Archivaldo Guzmán, Alfredo's older brother, is also among the kidnapped group.

Sources close to the family, meanwhile, told Proceso magazine that Iván was at the party, but escaped just in time. The authorities have also confirmed there was a seventh man who left the restaurant less than a minute before the attack talking on his phone, and later took a plane to an unknown destination.

Iván's fate is key to evaluating how serious the ramifications of the attack on the Guzmán family could be — both inside Chapo's Sinaloa cartel and external rivals.

"If it was also Iván, this becomes much worse," said leading security expert Alejandro Hope. "Because Iván is the smart one, Ivan is the successor."

Mexican drug trafficking has always tended to be a family business, and the Guzmáns are no different. The US treasury department includes three of El Chapo's sons — Iván, Alfredo, and Ovidio — on its Kingpin Designation list.

Alfredo also featured heavily in a Rolling Stone article published earlier this year and written by Hollywood actor Sean Penn about a secret mountain rendezvous with El Chapo while he was on the run between his escape from maximum-security prison in July 2015 and his recapture in January.

Penn depicts Alfredo as courteous and efficient as he transported the actor and Mexican telenovela actress Kate del Castillo from a hotel in Guadalajara, via car and plane, to the meeting. He was particularly impressed with Alfredo's very expensive watch.

The actor says Iván Archivaldo — who was waiting at the secret meeting — was "attentive" and possessed a "calm maturity."

The description of the two young Guzmáns clashes with the more common depictions of them as narco juniors prone to bouts of violent bravado.

Purported photographs show them holding large weapons, posing in front of flashy cars, and owning a pet lion. Iván was also a prime suspect in the 2004 murder of Kristen Deyell, a Canadian exchange student, and her Mexican companion, César Pulido, after a fight in a bar.

A leaked psychological profile of Iván drawn up after he was arrested for money laundering in 2005, and held for three years before his controversial release, describes him as "anxious, suspicious, reserved and evasive, with veiled hostility." The profile also highlights "probable psychological violence toward persons that he does not consider on his socio-economic level."

Iván, in particular, is also regularly singled out as a ruthless operator desperate to take over from his father who is currently awaiting near certain extradition to the United States. His ambition is sometimes cited as the trigger to an intense wave of violence in recent months in Sinaloa.

The other reason is a revival of an old feud with the Beltrán Leyva family cartel. The feud began when Chapo reputedly led the authorities to Allfredo Beltrán Leyva's safehouse. It took off when the Beltran Leyvas retaliated by the 2008 killing of another son of El Chapo — Édgar.

The Beltrán Leyvas lost the ensuing war and are much weaker today as a result. Recently, however, they are known to have formed alliances with the relatively new Jalisco New Generation Cartel. The authorities have already hinted heavily that the Jalisco cartel was behind Monday's attack

"If it is indeed Jalisco, this means probably all out war," said security expert Hope.