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

— -- Donald Trump Jr. and Paul Manafort have both agreed to negotiate with the Senate Judiciary Committee to provide documents "and be interviewed... prior to a public hearing" in regards to its Russia probe, according to statement from the office of committee Chairman Chuck Grassley.

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.

The announcement comes a day after senators threatened to subpoena the pair in pursuit of their investigation into Russian election interference. Both Grassley, R-Iowa, and committee ranking member Dianne Feinstein, D-Calif., had expressed confidence on Thursday in achieving cooperation with the president's son and former campaign chair.

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

On Wednesday, the committee invited the men -- who have come under scrutiny for their attendance at a June 2016 meeting at Trump Tower in which Trump Jr. believed they would receive incriminating information about Hillary Clinton -- to appear at a hearing next week and turn over documents related to their contacts with Russian nationals. They will not appear at Wednesday's session in light of their willingness to cooperate, according to an aide to Feinstein.

Glenn Simpson, founder of Fusion GPS, a research firm hired by Trump political opponents to investigate the GOP nominee's Russia ties, was also invited to next week's hearing, but declined, according to the statement from Grassley's office.

"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.