LOS ANGELES (AP) — U.S. immigration authorities are turning to smartphones to keep tabs on immigrants and ensure they attend their deportation hearings.

Increasingly, immigrants who are released from detention or stopped on the border are being required to snap a selfie or call to check in with federal agents using an app on their phones.

Advocates say it makes immigrants feel trapped even though some already paid bond to get out of detention.

They also worry about how U.S. officials might use data culled from the app known as SmartLink.

Immigration and Customs Enforcement declined to answer questions about the app.