Amnesty International is accusing oil giant Shell of complicity in human rights violations carried out by the Nigerian military in the 1990s.

In an 89-page report published Tuesday, Amnesty called for a criminal investigation of Shell after reviewing thousands of company documents and statements from witnesses. The documents demonstrate, Amnesty alleged, that Shell aided a Nigerian military campaign to silence protesters who sought to bring attention to the the effects of pollution in the oil-rich Ogoniland region.

Among the allegations in the report are claims that Shell operated a team of undercover police officers who were trained by Nigeria’s state security service. The unit was allegedly tasked with conducting surveillance on Ogoni activists resisting the exploitation of their land by corporate interests.

The report also asserts that Shell provided “logistical support” to the country’s military, which was responsible for the deaths of hundreds of Ogoni people in the early 1990s. The internal company documents, which HuffPost also reviewed, appear to show that Shell paid an “honorarium” as a “show of gratitude” to a military commander whose troops had engaged in a “bloody clash” with villagers several months prior. 

“The evidence we have reviewed shows that Shell repeatedly encouraged the Nigerian military to deal with community protests, even when it knew the horrors this would lead to – unlawful killings, rape, torture, the burning of villages,” Audrey Gaughran, Amnesty’s director of global issues, said in a statement.

“In the midst of this brutal crackdown Shell even provided the military with material support, including transport,” she continued, adding “and in at least one instance paid a military commander notorious for human rights violations.”

Shell has categorically denied the allegations.

Activists from the Movement for the Survival of the Ogoni People (MOSOP), a group spearheaded by Nigerian author and activist Ken Saro-Wiwa, had been lobbying the government for several years to grant Ogoniland political and economic autonomy. Their campaign lead Shell to announce in 1993 that it was ending operations in Ogoniland, citing security concerns, but the company ultimately moved forward with plans to build a new pipeline through the region.

Ensuing protests gave rise to a brutal crackdown by Nigeria’s military police, which, according to Amnesty’s report, included raids on Ogoni villages, torture and rape of Ogoni men and women, as well as the killings of roughly 1,000 people. In 1995, Saro-Wiwa and eight other Ogoni activists were hanged following an internationally discredited trial, which led to Nigeria’s suspension from the Commonwealth. In 2009, Shell agreed to pay a $15.5 million settlement after being accused of collaborating in the executions.

Amnesty alleged that Shell was aware of the military’s human rights violations in the 1990s and that a former regional head of security for the company had even provided information to the country’s internal security agency that aided in the crackdown.

Gaughran said Amnesty was “preparing a criminal file to submit to the relevant authorities, with a view to prosecution.”

Shell did not immediately respond to a request for comment. But in a statement sent to The Guardian, a representative for the Shell Petroleum Development Company of Nigeria Ltd (SPDC) said the company strongly denied the allegations.

“The executions of Ken Saro-Wiwa and his fellow Ogonis in 1995 were tragic events that were carried out by the military government in power at the time,” the spokesperson said. “We were shocked and saddened when we heard the news. Shell appealed to the Nigerian government to grant clemency. To our deep regret, that appeal, and the appeals made by many others … went unheard.”

The representative pointed to Shell’s company values of “honesty, integrity and respect for people” and called Amnesty’s claims “false and without merit.”

“SPDC did not collude with the authorities to suppress community unrest and in no way encouraged or advocated any act of violence in Nigeria,” the representative added. “We believe that the evidence will show clearly that Shell was not responsible for these tragic events.”

MOSOP, the Ogoni activist group, continues to have a presence in Ogoniland, which has experienced numerous oil spills over the years resulting in negative impacts on residents’ health and livelihoods. According to the local Nigerian news site, the Daily Post, the organization gave the federal government an ultimatum earlier this month, saying if it didn’t initiate a clean up of the region it would “face the anger of the people.”