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(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.”

After the lawsuit was filed, that mother and her daughter were reunited and a DNA test did confirm their relationship.

The letter from acting Homeland Security Inspector General John V. Kelly, which was provided to CNN, said his office has determined it will “conduct a review of this matter” and requested a follow-up meeting to discuss it further.

The issue of family units has been a source of difficulty for the department for years. A court ruling has held that children cannot be detained in what are essentially immigration jails for longer than three weeks, and the Obama administration thus issued guidance that family units would be released from custody together.

The Trump administration has decried this court ruling as a “loophole” that allows immigrants who have cleared the initial screening to pursue asylum protections in the US to live in the country for potentially years as their case works its way through the court system.

In a statement, DHS spokesman Tyler Houlton said the department has no policy of separating families “for deterrence purposes” but does follow laws protecting the interests of children at the border.

“Occasionally this results in separating children from an adult they are traveling with if we cannot ascertain the parental relationship or if we think the child is otherwise in danger,” Houlton said. “Unfortunately, we have seen many instances where human traffickers have used children to cross the border to gain illegal entry to our country as they know they are unlikely to be detained. This is one of the very loopholes we would like to see Congress end in order to gain operational control of our border.”