LAS VEGAS (Reuters) – Hackers attending this weekend’s Def Con hacking convention in Las Vegas were invited to break into voting machines and voter databases in a bid to uncover vulnerabilities that could be exploited to sway election results.

The 25-year-old conference’s first “hacker voting village” opened on Friday as part of an effort to raise awareness about the threat of election results being altered through hacking.

Hackers crammed into a crowded conference room for the rare opportunity to examine and attempt to hack some 30 pieces of election equipment, much of it purchased over eBay, including some voting machines and digital voter registries that are currently in use.

“We encourage you to do stuff that if you did on election day they would probably arrest you,” said Johns Hopkins computer scientist Matt Blaze, who organized the segment in a conference room at the Caesar’s Palace convention center.

The exercise featured a “cyber range” simulator where blue teams were tasked with defending a mock local election system from red team hackers.

Concerns about election hacking have surged since U.S. intelligence agencies claimed that Russian President Vladimir Putin ordered the hacking of Democratic Party emails to help Republican Donald Trump win the 2016 U.S. presidential election.

A Department of Homeland Security official told Congress in June that Russian hackers had targeted 21 U.S. state election systems in the 2016 presidential race and a small number were breached, but there was no evidence that any votes had been manipulated.

Russia has denied the accusations.

Jake Braun, another organizer, said he believed the hacker voting village would convince participants that hacking could be used to sway an election.

“There’s been a lot of claims that our election system is unhackable. That’s BS,” said Braun. “Only a fool or liar would try to claim that their database or machine was unhackable.”

Barbara Simons, president of advocacy group Verified Voting, said she expects Russia to try to influence the U.S. 2018 midterm election and 2020 elections. To counter such threats, she called for requiring use of paper ballots and mandatory auditing computers to count them.

More than 20,000 people were expected to attend the three-day Def Con convention.

The hacker voting village was one of about a dozen interactive areas where participants could study and practice hacking in fields such as automobiles, cryptology and healthcare.