Just before the start of the Democratic National Convention, top-secret emails from the Democratic National Committee were published on whistleblower website Wikileaks, in a major operation the FBI attributed to Russian hackers.

Some U.S. officials have raised subsequent questions: Were the hackers deliberately attempting to influence the election in favor of Donald Trump? Did Trump have any influence? And most importantly— if Russian hackers can breach the DNC internal network, what’s to stop them from hacking voting machines?

In a recent Washington Post piece, security technologist Bruce Schneier didn’t rule out the possibility of another attack— this time, disrupting the entire election system. “Voting machines have become computerized,” Schneier said in an interview with Boston Public Radio Thursday. “They use a touchscreen, they have a hard drive, there’s no physical paper, it’s all computerized.”

According to Schneier, the aftermath of the 2000 election introduced the Help America Vote Act, which helped states to buy new voting machines. “The problem is, like every other computer that we as a species have made, it’s vulnerable to hacking,” he said. “The worry is, these electronic voting machines can be hacked undetectably. In lots of audits over the past couple of decades of machines, every time security experts look at one, it’s incredibly vulnerable, the companies that made these machines try to keep the secret, there’s a lot of proprietary information… so we don’t actually know how secure these machines are. The worry that these computerized machines, electronic and voting machines can be hacked is serious.”

Bruce Schneier is a security technologist and a lecturer at the Kennedy School of Government. His latest book isData and Goliath: The Hidden Battles to Collect Your Data and Control Your World. To hear his full interview with Boston Public Radio, click on the audio link above.