This is the strangest thing …

As today's big Benghazi hearing gets underway, Republicans are saying Ambassador Thomas Pickering - the co-chairman of the State Department's committee investigating of the Benghazi attack - has refused to testify today.

The State Department says the opposite is true - that Pickering wanted to testify but Republicans would not let him.

"Ambassador Pickering volunteered to appear," a State Department official tells ABC News. "But [Government Oversight and Reform Committee Chairman Darrell] Issa said no."

"That is 100 percent untrue," says Issa spokesman Frederick Hill.

Related: New Testimony Expected on Benghazi Attack

In fact, Hill has released letters dated February 22 inviting Pickering ( read them here ), and the other co-chairman of the investigation, former Joint Chiefs Chair retired Adm. Michael Mullen, to testify at today's hearing.

"Ambassador Pickering initially told the Committee he was not available on that date," Hill tells ABC News. "When asked about a different date, he said he was not inclined to testify."

But the State Department says Pickering is ready to go right now - and happy to testify today.

"If Darrell Issa offers Pickering a chair, he will be there today," the State Department official tells ABC News. "The only reason Pickering isn't there is because Issa said no."

As the hearing was starting, we put that offer to the Committee.

"If Ambassador Pickering has reversed himself and wants to testify, we would welcome him at a future date," Hill said.

How about today?

Hill said it is too late. The Committee still has not received Pickering's offer to testify and, even if it comes now, the Committee has a "three-day rule" that requires witnesses be locked in three days in advance to give Committee members adequate time to prepare. Hill acknowledged that rule can be waived, but that won't happen today.

Bottom line: neither Pickering nor Mullen, who led the official State Department inquiry into the U.S. response to the Benghazi attack will be on hand at the hearing, at which security officers plan to say the State Department stood in the way of deploying additional security officers as the consulate in Libya came under attack.