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 authorized 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 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 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 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 email conversation that he has "nothin' to hide."
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 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 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 traveling to Mexico on his own dime, there is little doubt he was engaged in a journalistic endeavor when he met with one of the world's most wanted men, said George Freeman, director of the Media Law Resource Center, a non-profit group in New York.
"Being a movie star wouldn't disqualify him from the journalistic privilege," said 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 organizations have criticized. 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 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 the first amendment," said Theodore Boutrous, an attorney in California who represented Time Inc in a case that also involved the New York Times journalist Judith Miller. Miller 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 U.S. Circuit Court of Appeals, which has jurisdiction over Chicago has questioned whether any privilege exists for journalists in any context. Meanwhile, the 2nd U.S. 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.
David Schultz, an attorney who defends journalists and news organizations 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," said Schultz.