Washington (CNN) The Department of Homeland Security watchdog will investigate whether the Trump administration is improperly separating families in immigration custody, according to a letter the department's inspector general sent to the office of Sen. Dick Durbin, D-Illinois.

The inspector general will look into whether the agency is separating the children of asylum seekers from their parents, the letter says.

The review comes after Durbin led a coalition of Democrats in requesting the IG look into the matter after reports that DHS was separating children from their parents in immigration custody. While there have been specific reported incidents, it has been unclear if it is a widespread practice.

Homeland Security Secretary Kirstjen Nielsen testified last week before Congress that the department only separates adults from children in custody "in the interest of the child" -- for instance, if there's a suspicion of possible human trafficking or if they are unable to confirm the child is actually traveling with his or her parents or legal guardians.

She did, however, admit that in the case of a Congolese woman who was separated from her young daughter for months, which has spurred a lawsuit from the American Civil Liberties Union, that the process of verifying they were in fact family "took too long."

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