The United States says it is “deeply troubled” by what is calling the “horrific” and apparently “well-orchestrated” attack that killed at least 36 people Sunday in Burundi.

In a statement Tuesday, State Department spokeswoman Victoria Nuland condemned the violence and called for Burundi's government to make “every effort” to bring the perpetrators to justice and for a return to the reconciliation process there.

Burundi's president has declared three days of mourning and vowed to track down the gunmen, saying Burundi would solve the case within a month.

President Pierre Nkurunziza said the attackers are believed to have crossed the border from Congo, though authorities have not yet determined their identities.

Witnesses say the assailants burst into a pub near the Congolese border late Sunday, ordered patrons to the ground and began shooting. Officials say 23 people died at the scene and another 13 at nearby hospitals.

Burundi has been relatively peaceful since the last rebel army, the Forces for National Liberation, laid down 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 for all parties in Burundi to exercise restraint.