People who try to put Joaquin “El Chapo” Guzmán behind bars tend to end up dead.

It happened last year when a federal judge in Mexico who was involved in the case against the Sinaloa cartel leader was gunned down in broad daylight while he was out for a jog. It happened again in 2015 when the father of two high-level Sinaloa cartel informants in Chicago was murdered when he took a trip home to Mexico.

Now authorities in the U.S. are taking precautions to keep witnesses who plan to testify against the powerful drug lord alive long enough to appear at his trial in April.

Last week, Judge Brian Cogan granted a request from federal prosecutors to keep the identities of multiple “cooperating witnesses” secret from Chapo and his legal team until the last possible moment before they appear in court.

Cogan wrote in a December 14 court filing that he was “convinced that such an order is necessary to protect the cooperating witnesses” and said it would not prevent Chapo’s lawyers from preparing his defense, as they insisted it would. The judge wrote he was swayed by claims from prosecutors that Chapo had previously used “intermediaries, including family members, to threaten potential witnesses.”

Chapo’s lead attorney, Eduardo Balarezo, was livid over the decision, and suggested that federal prosecutors in Brooklyn had misled the judge into believing they were acting as messengers for the drug lord and attempting to tamper with witnesses.

“I have not passed on any messages from my client to third parties, period,” Balarezo said in a statement. “I certainly have not passed on anything that can remotely be perceived as as threat. The government is using the same playbook against me — making allegations without proof of wrongdoing — that it will use against Mr. Guzmán at trial. They are simply trying to chill the defense and prevent us from doing our job.”

Chapo has been held in extreme solitary confinement at a federal jail in Lower Manhattan since his extradition from Mexico in January. Under the jail’s “Special Administrative Measures,” the drug lord isn’t allowed to communicate with anyone other than his lawyers. He was allowed one visit with his sister, but Balarazo has said she was subsequently denied a visa and blocked from returning to the U.S. from Mexico for another visit.

Balarezo acknowledged in a partially redacted letter to the judge that his team “has made direct contact with various persons and requested to meet with them,” and said all but one of those individuals agreed to talk. Balarezo also said the defense has reached out to the Bureau of Prisons to schedule legal calls with federal inmates connected to the case, but they “have not spoken directly to any potential witness.”

The letter from Balarezo to Cogan included more references to the prosecution’s claims about alleged efforts by the defense to intimidate people who might testify against the Sinaloa cartel kingpin. “The government’s allegations concerning defense counsel are unfounded, troubling and in effect, allege a violation of the Special Administrative Measures imposed on Mr. Guzmán,” Balazero wrote.

Balarezo added that the “baseless allegation of defense counsel passing on messages to third parties that have been interpreted as a threat is nothing more than a not-so-subtle attempt at chilling the defense.”

Cogan was unmoved by those arguments, or by Balarezo’s claim that keeping the identities of witnesses secret would violate Chapo’s due process rights. The judge noted that prosecutors have said the information being withheld will be limited to “a few thousand pages of documents and a few dozen audio/video recordings,” which accounts for about 1 percent of the total evidence the government has against Chapo. Cogan said he “will not hesitate to adjourn the trial” if prosecutors surprise the defense with more material at the last minute.

According to court documents, the secret evidence includes “video or audio recordings of cooperating witnesses,” along with photographs and other materials that would make it easy for Chapo to figure out who is prepared to snitch. Balarezo proposed making the evidence available for “counsel’s eyes only,” but Cogan said he was skeptical that the defense would be able to make use of the materials without consulting Chapo.

The judge ruled that only certain witnesses will be allowed to keep their identities secret until the last minute. DEA agents and other law enforcement officials involved the case, for example, must be identified two months in advance of the trial.

Douglas Berman, a professor who specializes in criminal law and criminal sentencing at Ohio State University, said “the defendant has an absolute right to cross examine the witnesses against them,” but prosecutors could be bluffing about what they have on Chapo in order to make him take a plea deal.

“This becomes another layer of gamesmanship,” Berman said. “Prosecutors may both aggressive and sometimes sneakily play up how much information they have and say, ‘Well, we can’t tell you who is ratting you out because we have to keep the sources confidential, but we have 12 people who did transactions with you who are prepared to show up and testify. We got you dead to rights, plead guilty.’”

In Chapo’s case, prosecutors have said previously that they plan to call “dozens of witnesses who have had face-to-face dealings with Guzmán,” including “numerous Colombian cartel leaders and other suppliers” who arranged “multi-ton cocaine shipments” with him.

In January, the prosecution argued that Chapo should be detained and kept in isolation ahead of his trial because he has “a veritable army, ready to war with competitors and anyone Guzman deemed to be a traitor.” Prosecutors claim that “in addition to his cadre of armed guards, Guzman himself was known for carrying a gold plated AK-47 and a gold and diamond-encrusted .45 mm handgun.”

“Guzman also employed ‘sicarios,’ or ‘assassins,’ who carried out thousands of acts of violence, including murders, assaults, kidnappings, torture and assassination at his direction, to promote and enhance his prestige, reputation and position within the Sinaloa Cartel and to protect the Cartel against challenges from rivals,” prosecutors wrote. “Sicarios were deployed to silence potential witnesses and retaliate against anyone who provided assistance to law enforcement authorities against Sinaloa’s interests.”