A woman was detained while delivering Christmas gifts to migrant children in camps near the US-Mexico border, after officials suspected her of trying to smuggle ammunition.

Anamichelle Castellano was travelling across the southern border from Texas into Mexico on Monday with another volunteer and over 300 gifts, when she was stopped by Mexican authorities at the Gateway Bridge.

The gifts, from the migrant rights’ group Socorro Foundation, were meant for migrant children living in makeshift camps along the border.

Ms Castellano’s husband and nine-year-old daughter were following in another vehicle to help deliver the presents when she was instructed to lead her car through an X-ray machine at the border.

Mexican agents reportedly discovered a small box of bullets that were left in the vehicle from a previous hunting trip.

“Her husband had put them in there a long time ago and forgotten all about them”, Ms Castellano’s mother told KTRK.

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Her father also told the local news outlet the officials were “threatening her with federal prison … and federal charges” as she was detained with the other volunteer on Monday afternoon.

The two were reportedly later released on Christmas Eve and Ms Castellano was charged with possession of ammunition.

In an interview with KTRK, she said her husband and daughter waited all night in the jail’s lobby while trying to make sure she’d be released.

“Obviously this was a mistake, I mean, I’m dressed as Ms Claus, you know, I’m not — I’m here bringing gifts to children”, she said.

“Certainly they could see that this was not a criminal smuggling, you know, attempt. But unfortunately they didn’t see it like that and they detained us.”

Ms Castellano also said she later managed to bring necessities to the migrant camps and would later return with the presents where the vehicle was no longer detained by Mexican authorities.