Barack Obama has said congresswoman Ilhan Omar is under federal criminal investigation simply because she's an outspoken critic of the President Donald Trump.
"The full weight of the federal government is being brought down on a law-abiding elected official because she has dared to criticize the president," the former US president wrote on X, calling the probe "so appalling".
The White House defended the inquiry, saying the justice department was "focused on restoring law and order".
Last week, homeland security agents searched the Minnesota Democrat's Washington DC home as part of an investigation into her immigration status. Born in Somalia, Omar moved to the US with her family as a child.
US media outlets have reported that the probe is tied to her American citizenship and whether she lied about her age when she arrived in the US in 1995.
Questions have been raised about whether the age listed on Omar's citizenship application differed from her actual birth date.
Obama said in his X post on Sunday that her father became a naturalised citizen when she was 17, and not yet legally an adult, which then gave her citizenship.
"Now the Administration is actively trying to deport Congresswoman Omar, seemingly based solely on the unsubstantiated theory that she was actually 18 when her father was naturalized, and therefore is not a citizen," he wrote.
"If this becomes the new normal," he added, "there's no reason to believe it will stop with her - or that future presidents, in either party, won't do the same."
According to the US Citizenship and Immigration Services, a child born outside of the US automatically becomes a citizen when he or she has at least one parent who is a citizen by birth or naturalisation, or he or she is under the age of 18, and is a lawful permanent resident living in the custody of a citizen parent.
White House spokeswoman Lauren Bis denied the investigation was politically motivated.
"Unlike Joe Biden, President Trump has not weaponized the Department of Justice against his political foes," she said in an emailed statement to the BBC.
"The Department is focused on restoring law and order and keeping Americans safe."
Trump has often argued that federal criminal investigations against him as he campaigned for re-election were politically motivated.
The investigation into Omar could lead to the justice department seeking to "denaturalise" her, or revoke her citizenship. It would have to prove there were material lies in the naturalisation process.
In 2023, Republicans removed Omar from the House Foreign Affairs Committee for comments and tweets she made about Israel, which drew bipartisan criticism.
Omar has said that Trump has targeted her for years with "baseless lies and smear campaigns" and called the investigation "the latest escalation in his campaign of political intimidation".
But Trump has said the investigation had "nothing to do" with him, while describing Omar as a "terrible person" and a "complainer".
Federal agents reportedly seized her phone, computers, notebooks and passport in the raid a week ago.
Her lawyers have demanded the government return her belongings, CBS News, the BBC's US partner, has reported, arguing in a letter sent on Friday that the seizure of property had violated restrictions put in place by a judge to protect her work as a federal lawmaker.
The lawyers also alleged agents did not duly notify Omar or the House of Representatives general counsel that a search would be taking place and kept her from contacting her attorney for more than an hour, according to CBS.
Democrats characterised the raid as the president attempting to interfere with the legislative branch of government. Republicans, meanwhile, have said Omar should be treated the same as anyone else under the law.
When Obama first ran for president, and throughout his two terms in the White House, Trump repeatedly raised questions about his citizenship, asserting he was not born on American soil and was disqualified from serving in office. Obama released two records proving he had been born in Hawaii.
Obama did not mention that history in his X post on Sunday.