MEXICO CITY (AP) — Mexico's foreign affairs ministry has formally asked the United States government through its embassy for a thorough investigation into a Jan. 1 incident in which U.S. agents fired tear gas into Mexico to stop crossing migrants.

In a statement Thursday, the ministry said it lamented any violent acts along its border. It reiterated its commitment to the safety of migrants.

In the first hours of the new year, about 150 migrants attempted to breach the border fence in Tijuana. U.S. Customs and Border Protection said that agents fired gas at migrants throwing rocks. It said it was investigating.

But an Associated Press photographer at the scene only saw rocks thrown after agents fired gas as migrants scaled the fence.