WASHINGTON (Reuters) - A Democratic U.S. senator on Friday asked the FBI to probe whether Homeland Security Secretary Kirstjen Nielsen committed perjury when she testified to Congress that the administration never had a policy to separate immigrant families.

FILE PHOTO: U.S. Secretary of Homeland Security Kirstjen Nielsen testifies to a House Judiciary Committee hearing on oversight of the Department of Homeland Security on Capitol Hill in Washington, U.S., December 20, 2018. REUTERS/Joshua Roberts/File Photo

Senator Jeff Merkley has been an outspoken critic of the Trump administration’s moves last year to target immigrant families crossing into the United States for enforcement actions.

The administration implemented a ‘zero tolerance’ policy to criminally prosecute and jail all illegal border crossers, including those traveling with their children, leading to a wave of family separations last year.

The policy and disturbing images of young of children being held in cages sparked a public backlash, causing the administration to subsequently change course on family separation. President Donald Trump signed an executive order to end the practice on June 20, 2018.

In his letter to FBI Director Christopher Wray, Merkley noted emerging evidence that the Department of Homeland Security, as far back as December, 2017, had been developing a new policy to separate families.

Merkley released a memo on “Policy Options to Respond to Border Surge of Illegal Immigration,” which he said was created by senior Department of Homeland Security and Department of Justice officials discussing family separation as a deterrent for migrants.

Yet, the senator from Oregon noted, during testimony to the House of Representatives Judiciary Committee on Dec. 20, 2018, Nielsen said, “I’m not a liar, we never had a policy for family separation.”

The FBI declined to comment and DHS did not immediately respond to a request for comment.

The Office of Inspector General at the U.S. Department of Health and Human Services (HHS) said on Thursday it had identified “thousands” more separated children in addition to the 2,737 included as part of lawsuit challenging family separations brought by the American Civil Liberties Union (ACLU) last year.

The auditor said in a report that prior to the officially announced ‘zero tolerance’ policy, the government began ramping up separations in 2017 for other reasons related to a child’s safety and well-being, including separating parents with criminal records or lack of proper documents.

Trump campaigned for president on a promise to stop the flow of illegal immigrants and drugs over the southwestern U.S. border with Mexico. His demand for $5.7 billion to build a wall on the border with Mexico has led to the longest-ever partial government shutdown, now in its 28th day.