An aide to Secretary of State John KerryJohn Forbes KerryShould President Trump, like President Obama, forsake human rights in pursuit of the deal with a tyrant? GOP Senate report says Obama officials gave Iran access to US financial system Democrats conflicted over how hard to hit Trump on Iran MORE expressed frustration toward Rep. Darrell Issa (R-Calif.) for subpoenaing Kerry to testify before Congress about the Benghazi terrorist attack.

Kerry's spokeswoman Marie Harf told reporters she was upset that Issa issued the subpoena before inviting Kerry to appear before the House Oversight and Government Reform Committee on a voluntary basis, which is customary in congressional hearings.

"It is highly unusual for a subpoena to be issued before there's even an official invitation for testimony," Harf said, according to The Washington Examiner. "I think everyone can make their own judgments about that. I'm not sure the secretary's even aware of the request, again, given his travel. He's scheduled to be in Mexico on the 21st, which is the date that (Issa) has asked him to testify, which (he) would have known if they reached out to us instead of issuing a subpoena — I think by tweet, possibly."

Issa announced the subpoena Friday in a letter to Kerry.

"The State Department's response to the congressional investigation of the Benghazi attack has shown a disturbing disregard for the Department's legal obligations to Congress," the letter read.

Issa followed up with a series of tweets Friday, accusing the State Department of withholding Benghazi documents that he said could be embarrassing to the Obama administration if they were released.

"Today I announced a subpoena requiring Secretary Kerry to testify at a public @GOPoversight hearing on May 21," Issa tweeted.

The California lawmaker has been trying to get a hold of Benghazi documents that he says the State Department is either "withholding, has delayed, or has simply avoided searching for."

"The State Dept is not entitled to delay responsive materials because it is embarrassing or implicates senior officials," Issa tweeted.

But Harf said the secretary of State has nothing to hide about the events that took place surrounding the Benghazi attack. 

"The notion that there is something out there, that if Darrell Issa just keeps digging he can use politically, is just not borne out by the statistical facts of what we've done to look into what happened in Benghazi here," Harf said.

She said the issue has become personal for Issa.

"Well, he personalized it today with the secretary," Harf said. "I think he went there first."