The murder of Berta Caceres, 45, gunned down last year, highlighted the threat to Honduran activists and sparked international outrage.
Caceres opposed plans by the company Desarrollos Energeticos (DESA) to build a hydroelectric dam across a river on which indigenous communities depended.
"The existing evidence is conclusive regarding the participation of numerous state agents (police, military and officials) and senior managers and employees of DESA in the planning, execution and concealment of the murder," the 92-page report compiled by a five member panel of international experts said.
Caceres, a coordinator for indigenous organizations in Honduras, was assassinated in March 2016 by gunmen who entered her house in the community of La Esperanza. A Mexican activist, Gustavo Castro, was wounded in the attack.
The report said the public prosecutor's office had established that the murder was planned in November 2015.
At the time, indigenous and tribal communities were protesting against a project to build a dam on the Gualcarque River, on land belonging to the Lenca ethnic group.
The head of the Honduran Association of Renewable Energy, Elsia Paz, rejected the allegations and told AFP that DESA executives had presented evidence to the experts group that proved the company was "not linked to the crime".
She said the report was an attempt to influence elections next month in which Caceres' daughter is a candidate.
Eight people were arrested, among them an employee of Desarrollos Energeticos. But Caceres' family is demanding the masterminds be brought to justice.
Two Nobel Peace laureates, Tawakkul Karman and Sherin Ebadi, visited Honduras this month and called for the investigation into the murder to be deepened "to discovered all those who benefited from this crime."
The two women met Austra Berta Flores, the mother of Caceres, during the visit.
The expert group is comprised of academics and lawyers from the US, Guatemala and Colombia.
It made four trips to Honduras to interview more than 30 people as well as study evidence and reports from international human rights organizations.
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