(This Sept. 13 story corrects bureau’s name to Federal Bureau of Investigation in paragraph 6, not Federal Bureau of Information)

Hillary Clinton speaks at a campaign Voter Registration Rally at the University of South Florida in Tampa. REUTERS/Brian Snyder

NEW YORK (Reuters) - Two computer technicians declined to answer questions from U.S. lawmakers on Tuesday about the unauthorized private email system that Democratic presidential nominee Hillary Clinton used during her tenure as U.S. secretary of state.

Paul Combetta and Bill Thornton repeatedly invoked their constitutional right not to incriminate themselves during about 10 minutes of questioning while under oath before the House Oversight and Government Reform Committee. A third technician, Bryan Pagliano, declined even to appear, despite a subpoena ordering his testimony.

The investigation of Clinton’s email system has become a troublesome issue for her presidential campaign, with Republican rival Donald Trump saying a recent finding by federal investigators that she mishandled classified government secrets in her email should disqualify her from office.

Brian Fallon, a Clinton campaign spokesman, said in a statement it was up to the technicians whether to cooperate, “but we certainly are not prevailing upon anyone to cooperate with this sham of an inquiry.”

Clinton has said she regrets using the system in her New York home’s basement for work. Voters have said in surveys the decision contributes to a feeling she is untrustworthy.

Federal Bureau of Investigation director James Comey said in July there was evidence that Clinton and her staff may have broken the law with their “extremely careless” handling of classified government secrets, but concluded there were insufficient grounds to prosecute.

Combetta and Thornton work for Platte River Networks, the Denver technology firm that managed Clinton’s email servers soon after she left the State Department in 2013. They repeatedly invoked their Fifth Amendment protection from self-incrimination to each question from lawmakers. Jason Chaffetz, the committee’s Republican chairman, eventually excused them out of what he called “respect” for their rights.

Elijah Cummings, the committee’s most senior Democratic member, expressed sympathy for the technicians, saying the Republicans were only trying to use them to embarrass Clinton.

Chaffetz criticized Pagliano, who was paid by Clinton to manage her server while she was the nation’s top diplomat, for not complying with the subpoena and said the committee would decide after the hearing whether to punish him.

Pagliano was given a form of immunity from prosecution in 2015 for cooperating with the FBI as it looked into the mishandling of classified information over the server, but has otherwise cited the Fifth Amendment in declining to provide information to lawmakers. His attorney did not respond to questions.

A fourth witness, Justin Cooper, testified for more than two hours about his role setting up the email system in 2009 while he worked a Clinton family aide.

Clinton used an AT&T email account while a senator and a presidential candidate in 2008. Cooper said there was “a desire to change her email address because a number of people had received her email address over the course of those activities.”

A summary of the FBI’s findings showed Combetta saying he used software called BleachBit to delete an archive of Clinton’s work emails around the same time as a conference call with Clinton’s lawyers, despite lawmakers seeking the records through a subpoena.