Gunmen killed 36 people at a bar in Burundi on Monday.

The attack occurred at pub in the city of Gatumba, near the Congolese border. The proximity to the neighboring Democratic Republic of Congo (DRC) has led Burundian government officials to believe that the killers came from Congo.

The attackers were carrying guns and knives, and some of them were dressed in police uniforms, said one survivor, according to The Guardian.

They ordered everyone to lay down on the floor and started shooting the victims one by one.

Burundi is a tiny Central African nation of about 10 million people. It sits between Tanzania to the east and the DRC to the west. It is also directly south of Rwanda, a country which is no stranger to sectarian violence.

Burundi recently ended a bloody 15 year civil war, which caused the deaths of more than 250,000 people. The ethnic violence in Burundi between the Tutsi and Hutu finally stopped in 2009 (although a cease-fire was signed in 2005) and the country has been relatively calm since. However, elections in 2010 have sparked civic unrest, and some speculate that the violence Monday was a politically-motivated killing.

The border-city was also the location of the Gatumba Massacre in 2004. In August of that year, fighters from the Forces for National Liberation (FNL) -- the Hutu rebel group that attempted to overthrown the Tutsi-led government during the civil war -- rounded up and shot over 250 Congolese civilians in Gatumba. A reported 152 people were killed and more than 100 more were injured.

The FNL admitted responsibility for the attack, and later condemned it. In 2005, the group, which had been branded a terrorist organization by the African Union, denounced former leader Agathon Rwasa, who lead the group on a descent into hell.

So far, no group has claimed responsibility for Monday's attack. A Congolese military spokesman said that they attackers did not come from across the border.

Burundi's President Pierre Nkurunziza, a former university professor and Hutu soldier, said he is launching an investigation, and promised results in two weeks.