(CNN) — A brief trip to Canada and a small paperwork mistake landed a Washington state mother and her 6-year-old son in US immigration detention for more than three weeks, her attorney told CNN.

Sarah Shaw, a New Zealand citizen who has lived legally in the US since she arrived in 2021, was detained at the Blaine, Washington, Customs and Border Protection checkpoint when returning home after dropping her two oldest children off at the Vancouver airport for a flight to visit their grandparents in New Zealand.

Shaw, 33, chose the flight out of Vancouver because it was direct and she didn’t want her children to have to navigate a layover alone, her attorney Minda Thorward, told CNN.

But Shaw didn’t realize the travel permit that allowed her to exit and re-enter the US had expired. That’s

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