ATLANTIC CITY, N.J. (AP) — A hacker shut down four New Jersey Internet gambling sites for half an hour last week and threatened more cyberattacks over the holiday weekend unless a ransom was paid using the online currency Bitcoin, authorities said Tuesday.

David Rebuck, director of the New Jersey Gaming Enforcement Division, said Thursday’s attack was a so-called distributed denial of service attack, in which websites were flooded with information and requests for access that rendered them inoperative.

“The attack was followed by the threat of a more powerful and sustained attack to be initiated 24 hours later unless a Bitcoin ransom was paid,” Rebuck said. “This follow-up attack had the potential to not only negatively impact the targeted casinos, but also all business in Atlantic City” that share the same Internet service provider.

No ransom was paid. Rebuck said due to a response by law enforcement and casino staff, “the threat was mitigated with no significant disruption to service. All involved remain on heightened alert but are relieved that the holiday weekend has passed without incident. “

Rebuck did not identify the sites that were affected nor the size of the ransom sought. But he said numerous local and state law enforcement agencies are investigating.

Bitcoin is a virtual currency that, in addition to its legitimate uses, also has proven popular with online criminals.

No player money was stolen and no personal information was compromised in the incident.

New Jersey began Internet gambling in November 2013, and is one of three states in the nation that permit it, along with Nevada and Delaware.

It got off to a slower than expected start here but has shown signs of growth in recent months. New Jersey casinos won $122 million from Internet customers in 2014, its first full year of operation.