Border Patrol has special, secretive units that work to cover up any wrongdoing when agents kill someone or otherwise use force in potentially problematic ways, according to a letter sent to Congress on Thursday calling for an investigation.

These “shadow police units,” the letter says, have been operating since at least 1987 and without any actual authority under federal law. They do not appear in Department of Homeland Security documents as official entities.

The letter, written by the Southern Border Communities Coalition and Alliance San Diego, condemns the use of these units, which documentation shows have different names depending on their location, and suggests that the agents who worked for them could even be charged criminally with obstruction of justice.

A Border Patrol agent watches as a group of migrants walk across the Rio Grande on their way to turning themselves in upon crossing the U.S.-Mexico border in Del Rio, Texas on June 15. AP Photo/Eric Gay, File

The units “have allowed border agents to get away with nearly everything, including murder,” the letter says. It was sent to leaders of Senate and House judiciary, homeland security and oversight committees.

There are three entities with authority from Congress to perform criminal investigations. They are the FBI, the DHS Office of the Inspector General and the Customs and Border Protection Office of Professional Responsibility. These Border Patrol units are not among them.

“Their stated mission is to mitigate liability for agents,” said Andrea Guerrero, executive director of Alliance San Diego, quoting from a presentation created by San Diego’s Border Patrol unit and obtained by a journalist, John Carlos Frey. “In other words, their mission is protect agents, not the public.”

When asked about the units, a spokesperson with Customs and Border Protection who declined to be named on the record said that the agency has a “multi-tiered oversight framework in place to address allegations of misconduct involving agency personnel.”

“The U.S. Border Patrol maintains teams with specialized evidence collection capabilities across the southwest border,” the unnamed spokesperson said. “These teams consist of highly trained personnel available to respond around the clock to collect and process evidence related to CBP enforcement activities as well as critical incidents. In the case of serious incidents involving CBP personnel, members of these teams are sometimes called upon to assist investigators from CBP OPR and other local, state, and federal law enforcement agencies. This is a vitally important capability as many critical incidents involving CBP operations occur in remote locations where other agencies may be unwilling or unable to respond.”

The letter to Congress is the product of months of research and analysis, Guerrero said, and wouldn’t have happened without information that surfaced in a San Diego case.

She and other human rights attorneys investigating the 2010 killing of Anastasio Hernández Rojas at the San Ysidro Port of Entry found indications that the unit based in San Diego had tampered with and even destroyed evidence in the case to protect the agents involved. Then they began finding documentation of other cases in which similar units had interfered in other regions.

“This is not an isolated incident of bad actors in the case of Anastasio,” Guerrero said. “This is a systemic impunity problem that shows us that the impunity is not accidental. It is by design.”

Guerrero and her team found that the San Diego unit, called the sector’s Critical Incident Investigative Team, was the first to be notified after Hernández Rojas was beaten and tasered on the ground at the port of entry. The unit never notified San Diego police, who would have had jurisdiction over the criminal investigation. The local department found out about the incident after a media inquiry and arrived on scene a day later.

The Critical Incident Investigative Team controlled witness lists and were present at every interview during the San Diego police investigation, the letter says, and the team removed phrasing in the apprehension report for Hernández Rojas that said he was compliant before handing the report to San Diego police.

The Critical Incident Investigative Team was also present at the hospital soon after Hernández Rojas arrived and asked medical staff to draw his blood to test for drugs. Hospital records obtained by Guerrero’s team show that the blood was drawn but there were no results. The one blood test with results in his medical record indicates that there were no drugs detected. However, the letter says, the medical examiner in the case used a different blood sample — that had no chain of custody information — to determine that Hernández Rojas had meth in his body.

“It was this determination that led the Justice Department to decline to prosecute for murder,” the letter says. “This raises serious questions about [the unit’s] interference.”

These revelations are in addition to what Guerrero and team already published as part of their case with the Inter-American Commission on Human Rights — including that the Critical Incident Investigative Team illegally gave the hospital a subpoena for Hernández Rojas’ medical records and then refused to provide them to investigating police, and that the team sent the wrong video footage to investigators, allowing time for the actual recordings of the incident to be erased.

Former high-level CBP officials who worked in internal affairs, one of the entities authorized to investigate agents, have acknowledged and condemned the existence of these units.

“The purpose of an investigation is to collect the facts, regardless of whether they are exculpatory or not,” the letter says in quoting James Wong, former CBP deputy assistant commissioner for internal affairs. “Investigators should never set out to mitigate liability. That is inappropriate. Given that [the units] have worked to mitigate instead of collect facts leading to the truth shows that they are a questionable management tool, not a legitimate investigative tool. As such they should be abolished.”

The letter identifies several other cases in which such units have had access to and possibly tampered with evidence.

In the trial over the death of Jose Antonio Elena Rodriguez, a teenager who was standing in Mexico when he was shot by an agent — one of the only instances in which a Border Patrol agent was charged in a killing — the courtroom learned that the local Critical Incident Team had collected all of the evidence for the FBI.

“The Critical Incident Team, basically we went out and did a third-party investigation for the Border Patrol,” an investigator with the local Critical Incident Team explained to the court. “We investigated and collected forensic data in regards to shootings, uses of force incidents, collisions, things like that. Anything that the Border Patrol deemed to be critical and there might be some media attention or somebody was hurt, injured, or killed.”

The special unit, according to the letter, also shaped the investigation into the shooting of Marisol Garcia Alcantara, whom a Border Patrol agent shot in the head while she was sitting in the backseat of a car. She survived but still has bullet fragments in her skull.

According to a Nogales police incident report, the responding local police mostly directed traffic while the special Border Patrol unit and the FBI investigated. It is not clear which of the two entities gathered the evidence in that case.

Guerrero and her team also found that the Border Patrol considers participation in these kinds of units when promoting agents to supervisor positions. That means, Guerrero said, that many of the people higher up in the Border Patrol ranks likely worked on the units.