U.S. Customs and Border Protection agents fired tear gas and pepper spray into Mexico on Tuesday after 150 migrants attempted to cross a fence along the southwestern border, the agency said in a statement.

CBP said it was investigating the incident after the group attempted to travel into the U.S. near the border crossing in Tijuana shortly after midnight. The agency said officers fired smoke and other chemical agents into the crowd after some people began throwing rocks over the fence at border agents.

“To address the rock throwers assaulting agents and risking the safety of migrants attempting to cross who were already on the U.S. side, both smoke and minimal countermeasures were deployed,” CBP said in a statement. “Agents deployed smoke, pepper spray and CS gas [tear gas] to a position upwind of the rock throwers and south of the border fence.”

The agency said several teenagers were hoisted over the fence at the border and that officers saw some toddler-size children being lifted over the barbed wire barrier. Twenty-five people were arrested after crossing into the U.S., including two teenagers.

Mohammed Salem / Reuters A migrant from Honduras, part of a caravan of thousands of Central Americans, jumps from the border fence to cross into the U.S., in Tijuana, Mexico, Jan. 1.

A person who spoke to Reuters did not witness any rock throwing on Tuesday but reported that at least one migrant appeared to have been hit in the head by a tear gas canister.

The event is the latest encounter between Central American migrants and Border Patrol agents in recent months. In November, CBP said it fired tear gas into Mexico during a similar run on the border fence.

Last week, Secretary of Homeland Security Kirstjen Nielsen addressed the Christmas Eve death of an 8-year-old boy from Guatemala who was in the custody of CBP at the time by partly blaming migrant children’s parents for endangering their kids. The child was the second to die in agency custody in recent weeks; a 7-year-old girl died from dehydration while in Border Patrol custody in mid-December.

“Our system has been pushed to a breaking point by those who seek open borders,” Nielsen said in a statement before she headed to the border. “Smugglers, traffickers, and their own parents put these minors at risk by embarking on the dangerous and arduous journey north.”

Mohammed Salem / Reuters A migrant throws back a tear gas canister launched by U.S. Customs and Border Protection agents into Mexico, near the border crossing at Tijuana, Jan. 1.