Story highlights A June 2016 meeting has led the Senate judiciary committee to ask for testimony

The panel's chairman wants to talk to Donald Trump Jr. and Paul Manafort

(CNN) Senate judiciary committee Chairman Chuck Grassley threatened Thursday to subpoena both Donald Trump Jr. and Trump's former campaign chairman Paul Manafort if they do not appear before his committee for a scheduled appearance next week.

Trump Jr. and Manafort are scheduled to testify before the Senate judiciary committee on July 26 , the panel announced Wednesday, but neither Manafort nor Trump Jr. have publicly confirmed if they will appear.

"We sent the letter, I don't know whether they've accepted, but we sent the letter and we asked them for a voluntary appearance, and we also, I think we've indicated to them that -- I don't know if we said it in the letter or not -- but we indicated to them, at least publicly I indicated to them that there will be a subpoena if they don't come," the Iowa Republican told CNN.

Grassley added, "We are having a hearing next Wednesday, so obviously we want to hear right away so we can get the subpoena -- I hope they accept the subpoena voluntarily, but if they don't then you have to have a marshal give it and that takes a little more time."

The top Democrat on the committee, Sen. Dianne Feinstein of California, reiterated the subpoena threat as well.

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