Vicente Zambada's lawyers claim he and other cartel leaders were granted immunity by U.S. agents — and carte blanche to smuggle cocaine over the border — in exchange for intelligence about rival cartels engaged in bloody turf wars in Mexico.

Controversial defence: Vicente Zambada's claim could throw new light on how U.S. authorities deal with Mexican drug cartels

Experts scoff at the claim, which U.S. prosecutors are expected to answer in a filing Friday in federal court.

But records filed in support of his proposed defence have offered a peek at the sordid world of Mexico's largest drug syndicate, the Sinaloa cartel, which is run by his father, Ismael Zambada, and Mexico's most wanted man, Joaquin 'El Chapo' Guzman.

It's a world of brutality, greed and snitching, and federal agents would love to have the younger Zambada pass along more intelligence, especially if it could help bring down his family's operation or lead to the capture of Guzman, a billionaire who escaped from a Mexican prison in a laundry truck in 2001.

Main man: Vicente's father Ismael Zambada, godfather of the Sinaloa cartel, pictured at a party in 1993

'It comes down to whether he would be willing to give up his dad or Guzman,' said David Shirk, who heads the Trans-Border Institute at the University of San Diego.

'Would he be willing to give up his own dad? It seems unlikely.'

Zambada, 35, has rarely been seen since his 2009 arrest in Mexico City, after which Mexican authorities paraded him before TV cameras in a stylish black blazer and dark blue jeans.

His suave image was a sharp contrast to a photo of him with moustache and cowboy hat released by the U.S. Treasury Department in 2007.

He may have upgraded his look after he assumed control over cartel logistics in 2008 and, federal officials say, received authority to order assassinations. 

Mexico's most wanted man: Joaquin 'El Chapo' Guzman, who escaped from prison in a laundry truck

He was arrested and extradited to Chicago a year later to face trafficking conspiracy charges punishable by up to life in prison.

The Sinaloa cartel is one of Mexico's most powerful.

Named after the Pacific coast state of the same name, it controls trafficking on the border with California and is battling rival cartels in an effort to expand east along the 2,000-mile-long U.S.-Mexico border.

Accustomed to luxury in Mexico, Zambada has been held in a 10ft by 6ft cell in Chicago, is often served meals that have gone cold and hasn't been outside in 18 months, his attorneys say.

U.S. District Court Judge Ruben Castillo told the government on Thursday to file a response to those complaints.

Armed marshals led the shackled Zambada into Thursday's hearing.

He appeared at ease, even smiling and winking at a woman sitting on a spectators' bench.

Castillo will decide later whether Zambada's provocative immunity claim has any credibility, but many experts said they were skeptical.

'Personally, I think it is a bunch of malarkey,' said Scott Stewart, who analyzes Mexico's cartels for the Texas-based Stratfor global intelligence company.

'I mean, what the defence is saying is that a huge amount of cocaine was allowed to pass into the United States unimpeded.

'Why would you even have sought his extradition if there was this potential backlash?"

U.S. prosecutors briefly discounted Zambada's claim in one filing, but more details are expected in Friday's documents.

A spokesman for U.S. Atty. Patrick Fitzgerald would not comment on the allegation.

Neither would a Washington spokesman for the U.S. Drug Enforcement Agency, whose agents Zambada claims to have dealt with in Mexico.

However, clandestine intelligence deals are not uncommon, and conspiracy theories abound in Mexico about the government going easy on one cartel to keep the others under control.