Unknown assailants hurled an explosive device at a judicial complex in the northeastern Mexican city of Matamoros, leaving four injured, authorities said.

The bombing occurred minutes before 5:00 p.m. Tuesday at the federal judiciary branch facility, located in the city’s Victoria neighborhood, the Tamaulipas Coordination Group, or GCT, which comprises federal and state security forces, said in a bulletin.

Navy and Defense Secretariat personnel, Federal Police officers and Tamaulipas state police cordoned off the area to ensure that the injured, three of whom were inside the building at the time of the blast, could receive prompt medical assistance.

Navy personnel administered emergency first aid to one other injured person who was outside the complex at the time and was struck by bomb fragments, the GCT said.

All four of the injured were hospitalized.

An operation, meanwhile, is being carried out to locate the perpetrators of the bombing, the bulletin said.

Earlier Tuesday, a unexploded bomb was removed from a multi-sports facility in Matamoros, which is located across the border from Brownsville, Texas.

Army soldiers who arrived at the scene said there was never any risk of the bomb exploding, the GCT said in an earlier statement.

Tamaulipas has been a battleground between the Gulf and Los Zetas drug cartels for years, and the state is regularly among those with the highest number of homicides.

President Enrique Peña Nieto sent additional Federal Police and military personnel to Tamaulipas in May 2014 and ordered a thorough vetting of the state and municipal police forces to root out corrupt officers. EFE