"We knew there was a security breach in the data, and we were just trying to understand it and what was happening," said Josh Uretsky, reached by phone on Friday morning, a day after the campaign let him go.
He added, "To the best of my knowledge, nobody took anything that would have given the (Sanders) campaign any benefit."
The Democratic National Committee suspended Sanders' presidential campaign from accessing its voter database after it alleged the campaign took advantage of a software error to access Hillary Clinton's confidential voter information.
The Sanders campaign fired Uretsky over the breach. "That behavior is unacceptable and that staffer was fired immediately," Sanders' spokesman Michael Briggs said Thursday night.
Uretsky, who is experienced with the NGP-VAN system used by the DNC and has administered it before, said he first noticed the data breach on Wednesday morning.
"We investigated it for a short period of time to see the scope of the Sanders campaign's exposure and then the breach was shut down presumably by the vendor," he said. "We did not gain any material benefit."
According to the DNC, the Sanders campaign will remain suspended until it provides the DNC with a full explanation of the episode and provides proof that any improperly accessed data has been discarded.
Uretsky and his team notified people within the Sanders campaign of the breach on Wednesday and the news worked its way up the chain of command. After reporting it to the campaign, Uretsky said he intended to call the DNC to inform officials there. But before he could do that, the DNC called him.
"They called me fairly quickly after the breach was closed to inform me that there was something weird going on and that portions of the system were shut down," he said.
Uretsky says he got into the system to create a record to make it clear to anyone with NBG-VAN knowledge that he was "going through stuff that I wasn't supposed to have access to."
NGP-VAN is the technology company that provides campaigns access to the voter files.
The point: He wanted people with knowledge of the voter files to be able to clearly see that he was testing the depth of the breach.
"This wasn't the first time we identified a bad breach," he said, confirming to CNN that the Sanders campaign reported another breach to the DNC in October. "We reported it to them. They thanked us for reporting it and they told us the breach had been closed."
"In retrospect, I got a little panicky because our data was totally exposed, too," Uretsky said of how he handled the latest breach. " We had to have an assessment, and understand of how broad the exposure was and I had to document it so that I could try to calm down and think about what actually happened so that I could figure out how to protect our stuff."
Uretsky was informed at the end of the day on Thursday that his three-month stint at the campaign was over.
The now-former Sanders staffer would not comment on whether other members of his team were involved in testing the breach.
"It was 100% my responsibly and I take full responsibility for whatever happened," he said from Burlington before preparing to pack his life up and fly back to his hometown of Philadelphia.
The suspension comes at a terrible time for Sanders: One day after one of his strongest 24-hours of the campaign and one day before the third Democratic debate on Saturday. Without the data, it is near impossible for the Sanders campaign to target prospective supporters.