Two Mexican nationals in the U.S. on nonimmigrant visas were arrested during a traffic stop in Colorado last month while transporting 180,000 rounds of ammunition, authorities said Wednesday.

Caesar Ramon Martinez Solis, 41, and Humberto Ivan Amador Gavira, 24, both of Mexico, were pulled over in Canon City on March 26, the U.S. Attorney’s Office – District of Colorado said in a news release.

Two Fremont County detectives had spotted a white Chevrolet van passing them without dimming its headlights, in violation of state law, according to an arrest affidavit obtained by the Canon City Daily Record. The detectives said the van also failed to signal when turning into a gas station and had a defective license plate lamp.

During the traffic stop, the detectives discovered approximately 150 boxes of .308 ammunition and approximately 30 boxes of 7.62 ammunition, officials said. Each box was labeled as containing 1,000 rounds.

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Martinez Solis agreed to speak with U.S. Department of Homeland Security special agents and waive his right to an attorney, according to the affidavit.

Martinez Solis told authorities that he and Amador Garcia, whom he claimed was his brother-in-law, had traveled from Mexico to Denver a day earlier to purchase a vehicle before driving to Salt Lake City to look at another vehicle he was interested in buying.

In Salt Lake City, the men stopped at a firearms and ammunition store, where Amador Garcia bought the ammunition, according to the affidavit.

“[Martinez Solis] further explained that he did not know the intent with the ammunition but that he believed it was destined for Pueblo,” the affidavit stated. Pueblo is a city in Colorado located about 113 miles south of Denver.

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Martinez Solis added that the intended destination was on Amador Garcia’s phone, according to the affidavit. Authorities did not specify the destination.

Martinez Solis and Amador Garcia were both charged with Unlawful Possession of Ammunition by Alien Admitted Under a Nonimmigrant Visa.

The Denver Field Office of Homeland Security Investigations and the Fremont County Sheriff’s Office are investigating the case with help from the Denver Field Office of the Bureau of Alcohol, Tobacco, Firearms, and Explosives.

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Officials said the case is part of Operation Take Back America, a federal initiative designed “to repel the invasion of illegal immigration, achieve the total elimination of cartels and transnational criminal organizations.”