A special House panel investigating the Sept. 11, 2012, terrorist attacks on the U.S. consulate in Benghazi, Libya, has uncovered tens of thousands of new documents, but is still facing "obstacles" from the Obama administration, an interim report issued Monday said.

An "Interim Progress Update" was released Monday revealing the investigation "has uncovered new witnesses, new documents, new facts and will result in the most detailed and complete accounting of what happened in Benghazi."

And it reported that the panel in June will interview former Secretary of State Leon Panetta, Joint Chiefs of Staff Chairman Martin Dempsey and retired General Carter Ham, who was in charge of U.S. troops in Libya at the time of the attack.

The attack killed U.S. Ambassador Chris Stevens and three other Americans and has prompted questions about the role of then-Secretary of State Hillary Clinton.

Authored by panel Chairman Trey Gowdy, R-S.C., the report accuses executive branch officials of refusing to turn over requested emails and documents related to Benghazi.

Gowdy wrote that issuing subpoenas and otherwise using the courts to try to get the information would take too long and instead suggested the House use its "power of the purse" to persuade the executive branch to hand over the information.

"In addition to highlighting the public's right to know substantial pieces of information still to be disclosed, the House should also consider motivating the executive branch through the appropriations process," Gowdy wrote.

The panel has been engaged in a conflict with Clinton, who is running for president, over her willingness to testify before the panel in the coming weeks. Gowdy has requested that she appear twice before the committee in public hearings, but Clinton lawyer David Kendall wrote to Gowdy and said she will only appear one time.

Gowdy said the second hearing is needed to query Clinton about her use of a personal email server for State Department business and her decision to destroy 30,000 emails.

Gowdy said the timing of the hearing hinges on when the panel obtains thousands of documents it has requested but not yet received from the executive branch.

Gowdy said his panel also plans to interview Susan Rice, who is Obama's national security advisor, and Huma Abedin, a top aide to Clinton.