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A Mexican helicopter that crossed into Arizona and fired on US border agents on Thursday appears to have done so accidentally, a US government official told NBC News.

In a brief statement, Customs and Border Protection said that a Mexican helicopter crossed approximately 100 yards north into Arizona on Thursday morning. Two shots were fired from the helicopter, which was on a law enforcement operation.

The official described the rounds as “warning shots,” and said they were fired at two border agents sitting in an unmarked car. When the Mexicans realized what happened, they landed the helicopter and apologized profusely.

No injuries were reported. The incident is under investigation.

US and Mexican border authorities have been working on operations to stem human trafficking, the official said.

—Pete Williams