Updated 5:30 p.m. ET
Senate Judiciary Committee Chairman Chuck Grassley, R-Iowa, tells NPR that he's sending a letter to Donald Trump Jr., saying that he wants the president's eldest son to testify in an open session of the committee and will subpoena him if necessary.
Grassley first spoke with CNN.
It's the first formal invitation for Trump Jr. to appear before congressional investigators, who want to learn more about his June 2016 meeting with Russian lawyer Natalia Veselnitskaya.