Amnesty International says it has discovered five possible mass graves near Burundi’s capital, where a wave of political violence resulted in the deaths of scores of people last year.

The rights group said satellite images and video footage of disturbed earth seemed to verify witness accounts of mass graves in Buringa, on the outskirts of the capital, Bujumbura.

“These images suggest a deliberate effort by the authorities to cover up the extent of the killings by their security forces and to prevent the full truth from coming out,” said Muthoni Wanyeki, Amnesty’s regional director for East Africa, the Horn and the Great Lakes.

According to the Burundian army, at least 12 gunmen were killed in a series of coordinated assaults that targeted military bases in the capital on December 11, 2015.

In a crackdown that ensued, more than 87 people were reportedly killed, according to some sources.

UN human rights chief Zeid Ra’ad al-Hussein said the authorities responded to the attacks with house searches, arrests and alleged summary executions.

Earlier this month, the UN said it was also analyzing satellite images to investigate witness reports of at least nine mass graves, one of which is said to be containing more than 100 bodies, in and around Bujumbura.

The Central African country plunged into turmoil in late April 2015, when President Pierre Nkurunziza first announced his bid to run for a third consecutive five-year term. On July 21, last year, the electoral commission announced that Nkurunziza had won an outright victory in the presidential election.

The announcement was denounced by the opposition, which argued a third term by Nkurunziza was contrary to the constitution, which only allows two successive terms.

The violence that followed has, according to the UN, killed a total of at least 439; and 240,000 others have fled abroad.