Hillary Clinton’s personal lawyer has told a senate committee that emails and all other data stored on the Democratic presidential hopeful’s private server were erased before it was turned over to US government authorities.

In a letter sent last week to Ron Johnson, chairman of the senate homeland security committee, attorney David Kendall said the server was transferred to the FBI on 12 August by Platte River Networks, a Denver firm hired by Clinton to oversee it. The senate committee made Kendall’s letter public on Wednesday. In exchanges with reporters earlier this week Clinton said she did not know whether the data on her server had been erased.

Federal investigators, prompted by a request from the inspector general for the state department, requested custody of the server to learn whether the data stored on it was secure. NBC News has reported that an FBI team is now examining the server. Forensics experts said some emails and other data may still be extracted from servers even after deletion.

John Podesta, Clinton’s campaign chairman, told reporters on Tuesday in Columbia, South Carolina that to his knowledge no other copy had been made of the server’s contents other than what her lawyers had turned over to the FBI.

As campaign officials answered questions, one of Clinton’s rivals said the email issue had become a distraction for the Democratic party.

“I think that it’s a huge distraction from what we should be talking about as a party,” the former Maryland governor Martin O’Malley told reporters in Nevada.

Instead, he said, more debates should be held among the candidates to address raising the minimum wage, repairing the country’s infrastructure and other issues.

“Until we do, our party’s label is going to be the latest news du jour about emails and email servers and what Secretary Clinton knew and when she knew it.”

O’Malley said some people in the Democratic National Committee were “circling the wagons”.

Kendall, Clinton’s longtime personal lawyer, said in his letter to the committee that both he and another attorney at his firm were given security clearances by the state department to handle a thumb drive that contained about 3,000 emails – later turned over to the agency. He said the thumb drive was stored in a safe provided in July by the state department. Kendall did not say when he was given his clearance. The GOP-dominated senate judiciary committee has asked Kendall if he had any access to Clinton’s emails before he was given his security clearance.

Republican senators on both committees are pressing to see whether any emails sent or received by Clinton on the private server while she was secretary of state contained any secret information that should have been only exchanged on secured, encrypted government communications portals.

An inspector general for the state department said recently that several emails sent to Clinton did include such classified material – signaling that the transmission of those emails may have risked violating government guidelines for the handling of classified material.

Clinton campaign officials on Tuesday sought to show that the information contained in the emails she received did not risk spillage of classified data at the time they were sent to her. During a conference call, campaign aides pointed to a Fox News report that at least two of the emails that prompted the inspector general’s referral may have contained sensitive information but were not marked “classified” at the time they were sent to Clinton by aides.

Campaign spokesman Brian Fallon noted that the two emails were sent to Clinton from career diplomats, not political appointees, and they “did not have information marked ‘classified’ or any classified documents attached to them”.

One of the documents, a 2012 email to Clinton about arrests in Libya, was later classified as secret by the FBI, but then released in full this year by the state department, highlighting a dispute between the two agencies over whether the material should have been made public. A second email from 2011 was also released in full but reportedly contained classified military information.

“All this goes to show that when it comes to classified information, not all standards are black and white,” Fallon said.