Donald Trump invoked the Fifth Amendment to avoid answering questions over alleged fraud in his business empire, despite previously suggesting the manoeuvre was itself evidence of criminality.
It comes amid mounting legal pressures on the former US president, who on Wednesday was deposed in a fraud investigation into his family business two days after an unprecedented raid on his Florida home.
Mr Trump has previously ridiculed the use of the Fifth Amendment, which allows individuals to remain silent under questioning to protect against self-incrimination.
He once observed at a campaign rally that "the mob" takes the Fifth, asking: “If you’re innocent, why are you taking the Fifth Amendment?”
In a statement after his deposition at the New York attorney general's office on Wednesday, he said: "Now I know the answer."
"When your family, your company, and all the people in your orbit have become the targets of an unfounded politically-motivated witch-hunt supported by lawyers, prosecutors and the fake news media, you have no choice."
Mr Trump, 76, refused to answer questions under oath as part of a civil investigation into claims the Trump Organization misstated the value of his hotels, golf clubs and other assets to mislead lenders or tax authorities.
He said he had "no choice" but to decline to answer, describing himself as the victim of a "politically-motivated witch-hunt".
Mr Trump's decision to protect himself from self-incrimination took many by surprise and could negatively impact a potential lawsuit, given jurors in civil cases can draw a negative inference when a defendant invokes the Fifth Amendment.
But legal experts said any misstep in Mr Trump's deposition could have posed more significant jeopardy in a parallel criminal investigation under way by the Manhattan district attorney's office.
In explaining his decision, Mr Trump cited the Federal Bureau of Investigation's (FBI) search of his Mar-a-Lago estate - the first raid on an ex-president's home in US history.
"I have absolutely no choice [to invoke the Fifth Amendment] because the current administration and many prosecutors in this country have lost all moral and ethical bounds of decency," he said.
Earlier, Mr Trump had suggested that FBI agents could have planted evidence as they scoured his 124-room private club, revealing that his lawyers were not allowed to observe the search.
"The FBI and others from the federal government would not let anyone, including my lawyers, be anywhere near the areas that were rummaged and otherwise looked at during the raid on Mar-a-Lago," he said.
"Everyone was asked to leave the premises, they wanted to be left alone, without any witnesses to see what they were doing, taking or, hopefully not, 'planting'."
In his post on his Truth Social site on Wednesday morning, Mr Trump questioned why Democratic rivals Barack Obama, the former president, and Hillary Clinton, the 2016 presidential candidate, never came under the same scrutiny.
"Obama and Clinton were never 'raided', despite big disputes," he said.
The search marked a stunning and unexpected escalation of a Justice Department investigation into documents, including classified material, that Mr Trump may have taken with him to his Mar-a-Lago estate in Florida when he left office.
Several boxes were said to have been removed from the property on Monday night, reportedly including letters from Mr Obama, Kim Jong-un, and other correspondence with world leaders.
A source claimed every room in the mansion was searched, including Mr Trump's master bedroom and his wife Melania's wardrobe. Another group of agents combed through Mr Trump’s office, breaking open a safe.
Mr Trump has hinted the probes will encourage him to announce a highly-anticipated White House bid "sooner rather than later".
Republicans had previously urged Mr Trump to hold off on declaring his candidacy until after November's midterm elections, fearing an announcement could jeopardise candidates in swing districts.
But many now believe the FBI search could galvanise voters who believe Mr Trump is being politically targeted and potentially offer the Republican leader legal cover.
On Wednesday, Mr Trump told Republican congressmen that he “has made up his mind” about his long-teased run, and “enjoyed encouragement” from them to “get the decision out sooner rather than later”, revealed Jim Banks, a congressman for Indiana.
While the FBI's search of Mr Trump's home does not suggest that he faces imminent criminal charges, federal officials would have been required to demonstrate they have probable cause that a crime has occurred in order to obtain a search warrant.
The raid has led to frenzied speculation about a potential informant among Mr Trump's inner circle. According to the website Axios, some in his orbit have speculated the FBI might have received a tip-off from a close aide who provided details of what documents Mr Trump may have been storing at Mar-a-Lago.