MEXICO CITY — It was just before midnight when two men kicked in the door to Berta Cáceres’s house in the small Honduran mountain town of La Esperanza. Moving past the kitchen, one of them opened the door to her bedroom and fired six shots. She died moments later.
In a country where the fight to protect land rights provokes violent retaliation, the murder in March 2016 of another environmental defender might simply have receded into a grim tally of regrettable losses.
But Ms. Cáceres, 44, had won international acclaim for leading her indigenous Lenca community against a dam planned on their land. Her prominence transformed her killing into an emblematic crime — and turned the investigation that followed into a challenge to the entrenched impunity of the powerful in Honduras.
Now, 20 months after the killing, a team of five international lawyers has warned that the people who ordered it may never face justice.
The evidence, the lawyers said, points to a plot against Ms. Cáceres that was months in the making and reached up to senior executives of Desarrollos Energéticos, known as Desa, the Honduran company holding the dam concession.
“The existing proof is conclusive regarding the participation of numerous state agents, high-ranking executives and employees of Desa in the planning, execution and cover-up of the assassination,” the lawyers wrote.
Desa has repeatedly denied any involvement in Ms. Cáceres’ death or any connection to “acts of violence and intimidation.”
Eight suspects are in custody, including Sergio Rodríguez Orellana, the social and environment manager for the company, and Douglas Geovanny Bustillo, a retired Honduran Army lieutenant who was Desa’s director of security until mid-2015.
“What the public ministry has yet to do is indict the people who hired Bustillo to plan the operation,” said Miguel Ángel Urbina Martínez, one of the lawyers reviewing the case at the request of Ms. Cáceres’s family. The lawyers’ report, which The New York Times has obtained, will be released Tuesday.
The government’s investigation, by an elite unit in the Honduran attorney general’s office, remains open, although the lawyers’ group said there was no sign that it had progressed beyond the eight suspects.
Two American advisers, a retired homicide detective and a former federal prosecutor, have been working with Honduran authorities since the first days of the inquiry, part of an effort by the United States Embassy to push the government of President Juan Orlando Hernández to solve high-profile criminal cases.
Many of those cases involve powerful groups that critics say operate beyond the law. “The great challenge for Honduras is to dismantle these parallel forces,” said Mr. Urbina, a criminal justice expert from Guatemala and an adviser on judicial reform.
To prepare the report, Mr. Urbina’s group examined some 40,000 pages of text messages, which were retrieved by Honduran government investigators from three cellphones, one seized at Desa’s offices and two used by Mr. Rodríguez and Mr. Bustillo.
The messages, according to the report, show that the two men remained in frequent contact with three high-ranking Desa executives as they tracked the movements of Ms. Cáceres and other members of her organization, the Civic Council of Popular and Indigenous Organizations of Honduras, known as Copinh.
The conversations reveal, the lawyers said, that the orders to threaten Copinh and sabotage its protests came from Desa executives who were exercising control over security forces in the area, issuing instructions and paying for police units’ food, lodging and radio equipment.
“There was this criminal structure comprised of company executives and employees, state agents and criminal gangs that used violence, threats and intimidation,” said Roxanna Altholz, the associate director of the Human Rights Law Clinic at the University of California, Berkeley, and a member of the lawyers’ group.
The other members of the legal team are a former war crimes prosecutor, Dan Saxon, and two Colombian prosecutors who have tried human rights cases, Jorge E. Molano Rodríguez and Liliana María Uribe Tirado. They have been working on the case for a year, traveling to Honduras to conduct interviews and review case material.
The lawyers were chosen by Bertha Zúñiga, Ms. Cáceres’s daughter, with recommendations from the Center for Justice and International Law, a Latin American human rights organization.
The text messages were handed over to Ms. Cáceres’s family this May on the orders of a judge after Honduran prosecutors canceled four appointments to share their findings.
The question, Ms. Altholz said, was why the prosecutor’s office, which seized the phones in April and May last year, had failed to act on “the quality and the amount of information” that “it has had in its possession for the last year and a half.”
A spokesman for the attorney general’s office said he could not comment immediately.
For Ms. Cáceres’s daughter, the content of the messages only reinforces the sense that Desa’s executives felt untouchable. “They were so confident of impunity that they talked openly,” Ms. Zúñiga said.
The company has come to the defense of its employee, Mr. Rodríguez, the environmental manager. He is “a family man, honest and hard-working, who is unjustly deprived of his freedom,” Desa’s dam division, Agua Zarca Hydroelectric, said in an unsigned email. The company “completely trusts in the innocence of Mr. Rodríguez.”
Desa obtained a concession to build a dam on the Gualcarque River in western Honduras in 2009. By law the company was required to consult with the Lenca community, but Copinh opposed the project from the beginning, arguing that the dam would jeopardize the community’s water resources and livelihood.
From the start, Desa was an odd creation, said Juan Jiménez Mayor, the head of an anti-corruption commission backed by the Organization of American States. It had only $1,200 in equity when it won the dam concession, along with operating permits, water rights and a contract to sell power to the state electricity company.
In 2011, members of the Atala family, one of the most influential in Honduras, injected millions of dollars into the company and joined the board. Mr. Jiménez’ commission has begun to investigate Desa’s contracts, a move that drew an angry response from Honduran business groups.
Copinh fought the dam on several fronts. It filed legal challenges, led community meetings and brought a case before the Inter-American Commission on Human Rights, which ordered the Honduran government to provide protective measures for Ms. Cáceres. She had been receiving death threats and knew they were serious. Four members of Copinh were killed in 2013 and 2014.
In 2015, she won the Goldman Environmental Prize, which is awarded to grass-roots environmental leaders. But it was not enough to protect her.
In November 2015, according to the lawyers’ report, the former security chief Mr. Bustillo met with a high-ranking Desa executive. In January, he visited La Esperanza and later obtained a gun through Mariano Díaz Chávez, a former Honduran special forces officer who is accused of organizing the hit squad that assassinated Ms. Cáceres.
An attempt to kill Ms. Cáceres was planned for early February but called off, the lawyers said. “Mission aborted today,” Mr. Bustillo wrote to a Desa executive. “Yesterday, we couldn’t.”
The report did not name the Desa executives because they have not been charged by Honduran authorities.
Mr. Bustillo returned to La Esperanza for several days at the end of February and arranged to meet with the same executive on March 2. Early on March 3, after Ms. Cáceres was killed, Mr. Bustillo called him again.
After the killing, Mr. Rodríguez, the environment manager, forwarded details of the crime scene report that police had provided to one of the company’s executives.
“Sergio, relax,” another executive wrote through WhatsApp, a few days later. “Everything will come out O.K. you’ll see. Don’t panic and pass that on to other people.”