Burundi's president is vowing to track down gunmen who opened fire in a bar near the Congolese border late Sunday, killing at least 36 people.

President Pierre Nkurunziza said Monday the attackers are believed to have crossed the border from Congo, although authorities have not yet determined who was behind the massacre.

Witnesses says assailants burst into the pub late Sunday, ordered patrons to the ground and began shooting. The raid took place in Gatumba, about 15 kilometers west of the capital, Bujumbura and about five kilometers from eastern Congo.

Officials say 23 people died at the scene and another 13 at nearby hospitals.

After visiting the site, President Nkurunziza declared three days of mourning and vowed Burundi would solve the case within a month.

A local journalist tells VOA that the president said he might call for international help in solving the crime if required. He reports the president also said the attackers were now hiding abroad.

In an Associated Press report, Congolese military spokesman Sylvain Ekenge said he is “astonished” by claims the attackers came from his country. Ekenge said it is more likely they were part of Burundi's last rebel army, the Forces for National Liberation .

Burundi has been relatively peaceful since the FNL laid down its arms in 2009 after 20 years of insurgency. But recurring attacks have raised fears of a return to conflict. The government has blamed a number of previous attacks this year on armed bandits.

In a statement Monday, U.N. Secretary-General Ban Ki-Moon condemned the violence, calling it senseless, and called on all parties in Burundi to exercise restraint.