Hackers are demanding increasingly hefty ransoms to free computers paralyzed with viruses, as cyber criminals seek to maximize profits from large numbers of victims willing to pay up, according to cyber security firm Symantec Corp.

The average demand embedded in such malicious software, which is known as ransomware, more than tripled last year to $1,077 from $294, and the pricing has continued to rise in 2017, according to Symantec.

“The bad guys haven’t found the top end of what people will pay,” Symantec Director of Security Response Kevin Haley said in a telephone interview.

Symantec said 69 percent of ransomware infections in 2016 hit consumer computers, with the remainder targeting businesses and other organizations.

More than a third of consumer ransomware victims around the globe pay cyber criminals to regain access to their data, according to Symantec. In the United States, where such attacks are most prevalent, 64 percent pay.

“If six out of ten people will pay your ransom when it’s three hundred bucks, you’re thinking ‘What if I raise it to four hundred? What if I raise to five hundred?'” Haley said.

The surge in cyber extortion has been fueled partly by the sale of ransomware kits, which sell for $10 to $1,800 on underground markets and make it easy for wannabe cyber crooks to get in the business, according to Symantec.

One kit, known as Shark, lets users name their demand, which its creators collect from victims and pass on to attackers, minus a 20 percent commission.

Ransomware attacks have increased sharply over the past year, with criminals targeting hospitals, police departments and other providers of critical services in the United States and Europe.

In some cases, the attacks have interrupted critical public services.

U.S. and European hospitals have been forced to divert patients to other facilities when ransomware paralyzed computer systems.

Local police have been forced to manually dispatch calls, and San Francisco’s public transit system was unable to collect fares for a weekend during the busy Christmas shopping season.