WASHINGTON—Kash Patel, a close associate of former President Donald Trump, is set to soon testify before a federal grand jury probing the handling of classified documents at Mar-a-Lago after receiving immunity for his information, people familiar with the matter said.A federal judge recently decided the Justice Department couldn’t force Mr. Patel to testify without such protection against his statements being used against him in some future prosecution.That ruling, the people said, opens the door for Mr. Patel, who says Mr. Trump broadly declassified White House documents while still president, to answer questions.Continue reading your article witha WSJ membershipa WSJ membershipAlready a member?- Best Buy:Save 15% or more on the Best Buy deal of the Day- Expedia:Today's Expedia promo code: Extra 10% off your stay- Journeys:Fall Sale: 50% off select styles + free shipping- Nike:60% off running shoes and apparel at Nike"