The IT specialist who ran Hillary Clinton’s private email server ignored a subpoena to testify before Congress on Tuesday, leaving GOP lawmakers fuming and threatening retribution.

Two other witnesses linked to Clinton’s email server invoked their Fifth Amendment rights and repeatedly refused to answer questions.

Bryan Pagliano, a 2008 Clinton presidential campaign staffer who later went to the State Department, was a no-show at a House Oversight Committee hearing where he was scheduled to be one of four witnesses.

“It’s a serious matter,” said committee chair Rep. Jason Chaffetz (R-Utah).

“Mr. Pagliano has chosen to evade a subpoena duly issued by the committee of the United States House of Representatives. I will consult with counsel and my colleagues to consider a full range of options available to address Mr. Pagliano’s failure to appear.”

He pointed out that Pagliano had earlier cooperated with FBI probes of Clinton’s emails after getting immunity.

“You’d think somebody would sing like a songbird if you got immunity from the FBI. What are you afraid of?” asked Chaffetz.

Pagliano had invoked Fifth Amendment privileges last year when asked to testify before the House Benghazi Committee.

His lawyers claimed he is being harassed “for unvarnished political purposes.”

Two other witnesses, Paul Combetta and Bill Thornton of Platte River Networks, which helped manage the email server, repeatedly invoked their Fifth Amendment rights.

Combetta took the Fifth six times and Thornton took the Fifth four times, before both were excused.

Democrats blasted Republican leadership for using taxpayer dollars by calling a third committee hearing within four business days on Clinton’s emails.

“Enough of these emails! I think those of us who have sat through this hearing can say the same,” said Rep. Carolyn Maloney (D-Manhattan), invoking a line used by Sen. Bernie Sanders during the primary campaign.

The only witness who actually answered questions was Justin Cooper, a longtime Bill Clinton aide who registered the clintonemail.com domain name and brought Pagliano on board to manage the private email server.

Cooper said he did not have security clearance although he was an administrator, along with Pagliano, for the server.

In his administrator role, Cooper said that he was notified whenever there were failed multiple log-in attempts — including by unauthorized users who tried to get into the server “with some frequency.”

He also said that Clinton’s old BlackBerrys were sometimes destroyed with a hammer after he transferred data to a new device.

“Why did you use that kind of method? It seems somewhat, if you will, barbaric. A hammer to a phone?” asked Rep. Buddy Carter (R-Ga.).

“I think it’s practical to not just throw away a cold device into some sort of garbage receptacle where someone might pick it up out of curiosity and try to use it,” Cooper responded.