Trump Jr., Manafort agree to cooperate with Senate panel and won't attend Wednesday hearing, Grassley says

The Senate Judiciary Committee invited them Wednesday to a hearing next week.

Representatives for both Trump Jr. and Manafort did not respond to requests for comment from ABC but previously said they are cooperating with the Congressional investigations.

“I’m not concerned, because if they don’t they will be subpoenaed," said Feinstein.

"A subpoena has been issued to compel his attendance," the statement said.

"Simpson's attorney has asserted that his client will invoke First and Fifth Amendment rights in response to the subpoena," continued the statement.

While Trump and Manafort will be cooperating with the committee, the statement adds that the panel "reserve[s] the right" to issue subpoenas for each in the future.

Editors' Note: An earlier version of this story indicated that representatives for Donald Trump Jr. and Paul Manafort had confirmed negotiating with the Senate Judiciary Committee to ABC News. They had previously said to ABC News that they'd cooperate but didn't respond on this specific matter.

ABC News' John Santucci contributed to this report.