NEW YORK/WASHINGTON (REUTERS) - US investigators will examine actor Sean Penn's interactions with Mexican drug lord Joaquin "El Chapo" Guzman, two US government sources said on Monday (Jan 11), but it is unclear if prosecutors would try to force the actor to turn over information about his interview with the recaptured drug kingpin.

Mexico is pressing the US government, which has requested Guzman's extradition, to find out more about Penn's dealings with the infamous head of the Sinaloa drug cartel, according to one US government source who requested anonymity because he was not authorised to speak publicly about the case.

Rolling Stone Magazine rushed Penn's 10,000-word article to publication on Saturday after Mexican officials captured Guzman in a dramatic raid, ending a months-long manhunt following his July escape from a maximum security prison.

The only interview the drug lord is believed to have given in decades was brokered with the help of the Mexican television star Kate del Castillo.

Mexico's Attorney-General on on Monday said her office has an open line of investigation into Penn's meeting with Guzman, saying their rendezvous - captured by Mexican surveillance - was an "essential" element in the drug lord's arrest.

If the US authorities ultimately subpoena Penn or want him to testify against Guzman, it would be difficult to force the actor to reveal facts beyond the published interview, since he could invoke "journalistic privilege", which in some cases protects reporters from divulging information about their work, said lawyers with expertise in US media law.

The US government sources could not confirm whether or not the authorities will subpoena Penn. The request to extradite Guzman to the United States to face federal charges is still at a very early stage and Mexico said the process could take years.

The question of journalistic privilege has been a hotly debated topic in the United States, with federal courts disagreeing on how much reporters are protected.

Representatives for Penn did not respond to a request for comment.

The actor told the Associated Press in a brief e-mail conversation that he has "nothin' to hide".

Mr Mike Vigil, former DEA chief of international operations who has been briefed on the Guzman investigation by US and Mexican law enforcement, said there is a "very strong possibility that Penn and Del Castillo are going to have to testify".

Beyond requiring Penn to testify or hand over information, it would be extremely unlikely that the US authorities would have grounds to bring criminal charges against Penn himself, the sources said.

Unless Penn aided and abetted Guzman in some way, the Oscar-winning actor would not have a duty to disclose to the authorities that he was talking to a fugitive, legal experts said.

- MOVIE STAR AS JOURNALIST -

Even though Penn is a well known actor and was travelling to Mexico on his own dime, there is little doubt he was engaged in a journalistic endeavour when he met one of the world's most wanted men, said Mr George Freeman, director of the Media Law Resource Centre, a non-profit group in New York. "Being a movie star wouldn't disqualify him from the journalistic privilege," said Mr Freeman.

Of particular interest to investigators could be Penn's revelations that the drug lord discussed "a host of corrupt major corporations" that helped Guzman launder money from his vast criminal enterprise. The actor said he agreed not to publish the company names.

A disclaimer on the article says it was submitted to Guzman for approval before publication, a practice that several renowned journalists and professional organisations have criticised. The government could try to argue Penn's article is not journalism and therefore not protected, but that argument would be difficult to win, said Mr Freeman.

Prosecutors might also try to say the actors were not working as a journalists because Guzman's original interest in connecting with Del Castillo was for a movie deal, according to Penn's account.

"You would have to know all the facts, but making a movie is creative activity and even if it is part of a business deal it would be protected by first amendment," said Mr Theodore Boutrous, an attorney in California who defended New York Times journalist Judith Miller, who went to jail for refusing to testify in a case.

Depending on where Guzman is tried, it could make a difference in how the courts would treat potential testimony from Penn.

Guzman has been charged in seven separate US indictments stretching back to 1995 and but Chicago and New York are the leading contenders to host the high profile trial, according to former US law enforcement officials.

The 7th US Circuit Court of Appeals, which has jurisdiction over Chicago, has questioned whether any privilege exists for journalists in any context. Meanwhile, the 2nd US Circuit Court of Appeals, which handles cases out of New York, has ruled that the party requesting a subpoena must prove the confidential information sought from a journalist is both highly relevant to the case and not attainable through any other source, raising the bar for prosecutors.

Mr David Schultz, an attorney who defends journalists and news organisations at Levine, Sullivan, Koch & Schulz in New York, said the split in the courts raises more questions than answers.

"Whether or not he would have any privilege at all is pretty much in the air," he said.