(CNN) Nearly 300 undocumented immigrant parents may have been separated from their children months before the Trump administration's "zero tolerance" policy along the border took effect, and officials did not keep a full record of separations, Customs and Border Protection admitted in a letter released Monday.

The separations took place in west Texas in 2017, Customs and Border Protection Commissioner Kevin McAleenan said in a letter to Sen. Ron Wyden, D-Oregon.

"[F]ollowing an increase in family unit apprehensions in El Paso, US Border Patrol's El Paso Sector undertook a limited effort to pursue prosecutions against all amenable adults, including parents in family units," McAleenan wrote. "CBP records indicate that this led to approximately 283 parents referred for prosecution between July and November 2017."

Bringing criminal charges against parents for unlawful entry required separating them from their children.

A DHS official previously confirmed to CNN that the agency had tested the policy of prosecuting parents caught illegally crossing the border in the El Paso sector.

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