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SAO PAULO (Reuters) - Brazilian authorities said on Thursday they were investigating a police raid that ended with 10 land activists killed in the Amazon region, the deadliest such conflict in over two decades.

Nine men and one woman were killed when police arrived at the Santa Lucia farm on Wednesday, which a group of landless activists that included 150 families had invaded two years ago.

Para state security officials said in a statement the police were fired upon as soon as they arrived. No officers sustained any injuries during the conflict. Leaders of the land activists were not immediately reachable for comment.

Para state police said in a statement that they were acting on a local judge’s order to remove the families from the private land, and also carry out 14 arrest warrants in connection to the murder last month of a security guard employed by the ranch owner.

Federal and state prosecutors said they were investigating the killings. Police said they, too, had started an inquiry into the officers’ actions.

Landless activists routinely invade massive ranches and farms in Latin America’s largest nation, where there is deep inequality in land distribution. Brazilian law allows for landless activists to occupy and eventually take ownership of land determined to be not actively used for agricultural purposes.

For decades, the fight over Brazil’s vast tracts of land have repeatedly led to murders, most often by rich ranchers and farmers’ hired gunmen, federal prosecutors have said.

The Catholic Church’s Pastoral Land Commission, which tracks the conflicts, said in an emailed statement on Thursday that 61 people were killed in such rural conflicts last year, the most since 2003. The group said 26 people have been killed in such conflicts this year.

Wednesday’s deaths were the most in a land conflict since 1996, when Para state police shot and killed 19 land activists who had blocked a highway to protest their right to remain on a ranch that 3,000 families had occupied.