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An international team of lawyers said Tuesday that the 2016 killing of Honduran environmental activist Berta Caceres was the product of a coordinated plot that began four months earlier and suggested that leadership of a dam development company may have ordered her assassination.

A report by the International Advisory Group of Experts, or GAIPE for its initials in Spanish, said the group had identified "possible intellectual authors" of the murder beyond the eight individuals already charged and concluded the killing was "not an isolated incident." But it warned that the investigation has been clouded and the full truth may never be brought into the light.

Caceres, who was awarded the prestigious Goldman Environmental Prize for leading her Lenca indigenous people in opposing a hydroelectric project, was slain March 2, 2016, by gunmen who forced their way into her home in the middle of the night. Gustavo Castro Soto, a Mexican activist, was wounded.

GAIPE, made up of five lawyers from the United States, Colombia and Guatemala, formed last year to conduct an outside review at the request of Caceres' family and her activist group, the Civic Council of Popular and Indigenous Organizations of Honduras, who feared authorities would not bring to justice those behind her killing. Its findings do not carry the weight of law.

The report was based on dozens of interviews, criminal case files, independent reports and what the group described as partial access to evidence provided by government investigators, including "a fraction of" the phone records that are at the core of the case.

"Despite the secrecy of the Public Prosecutor's investigation, GAIPE has been able to establish the participation of executives, managers and employees of (dam developer) DESA, of private security personnel hired by the company, of state agents and parallel structures to State security forces in crimes committed before, during and after ... the day of the assassination," the report said. "Those crimes remain unpunished.

Spokesman Yuri Mora-Carias of Honduras' Public Ministry, which is in charge of prosecutions, told The Associated Press that the eight under arrest are considered the material authors of Caceres' killing and the murder weapon has been recovered, but the investigation remains open.

"What has been difficult for the Public Ministry is getting to who paid for the assassination," Mora-Carias acknowledged.

He added that an anti-corruption mission sponsored by the Organization of American States is assisting and authorities hope to identify all who were involved.

Prosecutors have not publicly linked DESA, or Desarrollos Energeticos SA, to the killing or ascribed a motive to the suspects.

DESA has repeatedly denied involvement in Caceres' murder and says the anti-dam protests organized by Caceres and her organization, known as Copinh for its initials in Spanish, were violent and damaged property.

GAIPE did not accuse any specific DESA executive of involvement in the killing, and the names of people not already charged were redacted from transcripts of communications. No company executive has been arrested.

Honduras is the world's deadliest country per capita for land activists like Caceres, with 14 slain last year, according to a July report by Global Witness. Two other members of Copinh were among those killed in 2016.

Citing communications between DESA employees and government officials, GAIPE's report said the company allied from the beginning with state and private security entities on a strategy to "control and repress" those protesting the project.

That included surveilling Caceres, sometimes paying informants for tips about her activities, it said. DESA also allegedly paid journalists to publish "disinformation" about her and Copinh, which opposed the development on the grounds that it threatened ancestral lands.

"The strategy was to control, neutralize and eliminate any opposition," the report said.

GAIPE called Caceres' killing "the product of a plan structured by top executives of DESA, who involved employees of the company in actions of surveillance and vigilance ... (and) acted in alliance with State security forces and parallel structures."

The report alleged that in November 2015, a top DESA official contacted Douglas Geovanny Bustillo, a security worker with a military and government background, to put Caceres' murder in motion.

Bustillo is among the suspects who have been arrested and charged. He and others allegedly first tried to kill Caceres on Feb. 5-6, 2016, but aborted the plan because they lacked a vehicle and because there were other people with her.

According to the report, phone records show that on March 1, 2016, Bustillo and an unnamed top DESA official scheduled a meeting for the following morning. There was no more mention of the meeting, but on March 2, it said, suspects were in the Caribbean coastal city of La Ceiba before traveling to La Esperanza, which is about a six-hour drive.

There, at least two gunmen kicked in a door on the back patio of Caceres' house shortly before midnight. She was shot three times in the chest.

Records show multiple communications between the suspects before and after the killing, the report said. It said that around 6:30 a.m. the day after the slaying, one of the suspects contacted a phone number attributed to the unnamed DESA official. According to the report, Bustillo had contacted the same official Feb. 6, after the first planned attack was scuttled, with the message: "Mission aborted today. Yesterday it could not be done. I await your instructions because I have no logistics I am at zero."

GAIPE said there have been some positive developments in the investigation such as the arrests and the recovery of the murder weapon. But it said it has documented numerous instances of error and negligence.

The official investigation centered at first on members of Copinh and associates of Caceres and attributed the killing to a crime of passion. GAIPE also said police first inspected the home nearly three hours after the murder and "noted the presence of private individuals in the house and the modification of the crime scene."

"It has been insufficient and incomplete," the report said of the investigation, raising concerns about whether the full story will ever emerge. "The totality of people responsible has yet to be identified."

GAIPE called for all evidence, including cellphones, USB drives and computing equipment, to be handed over to the victims' lawyers. Some phones seized as part of the investigation were said to have yielded no information for reasons such as they "didn't have a battery" or "were in bad shape, the report said, suggesting a halfhearted effort to retrieve data. The group also said it is aware of other intercepts whose content has not been made public.

While the report did not accuse any executive by name, two of the people charged in the murder were current or former employees of the company: Bustillo and Sergio Rodriguez, DESA's environmental, social and communications manager. And GAIPE said DESA leadership was in close contact with those monitoring Caceres.

"From the information in the case file it can be concluded that both Sergio Ramon Rodriguez Orellana and Douglas Geovanny Bustillo participated, under the direction of top officials from the DESA company, in the attack" on Caceres and Castro, according to the report.

The report faulted Honduran security forces for failing to protect Caceres despite the known risk to her life after repeated harassment and threats.

GAIPE also faulted international lending institutions, saying that while they pulled financing for the Agua Zarca dam project in the months after Caceres' killing, they shouldn't have backed it in the first place because of numerous red flags.

In a statement Caceres' mother praised the report, which comprises nearly 90 pages of text, footnotes and annexed material.

"Now it is possible for the authorities, the Public Ministry and prosecutors to take notice and punish the true killers. ... May we not rest in seeking justice for Bertita," Austra Flores said.

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Associated Press writers Christopher Sherman in Tegucigalpa, Honduras, and Martha Mendoza in San Francisco contributed to this report.