In April, former Obama national security adviser Susan Rice was discovered to be the one behind the unmasking of Trump transition officials, who happened to be caught up in “incidental surveillance.”

“Susan Rice, the former National Security Advisor to President Obama, is refusing to testify before a Senate Subcommittee next week on allegations of unmasking Trump transition officials. Not good!” President Trump tweeted out about the revelation.

On Thursday, Rep. Trey Gowdy expressed his intent to take action, saying that a subpoena may have to be used to bring Rice before Congress.

“There are things called subpoenas. You shouldn’t have to use it with a former national security adviser but if you do, you do,” said Gowdy, calling Rice a “very important witness.”