The company's corporate site, as well as the home pages of the Italian-themed Venetian and Palazzo casinos in Las Vegas, displayed a screen Wednesday that said they were down for maintenance. The message provided phone numbers for all Sands properties, but not emails, because the hacking knocked that system out too.
Patrons can still make reservations through third-party sites.
Sands spokesman Ron Reese declined to say whether the company is aware of credit card records being stolen.
"While we have been able to confirm that certain core operating systems were not impacted by the hacking, the company remains focused on working through a step-by-step process to ascertain what, if any, additional systems may have been impacted," Reese said in a statement.
FBI spokeswoman Jenny Shearer said the FBI and Secret Service were investigating. The Secret Service is charged with safeguarding the country's financial systems.
The Nevada State Gaming Control Board was also investigating the cyber-attack.
Las Vegas Sands Corp. runs the largest casino in the world in the Chinese gambling enclave in Macau. It also owns hotel-casinos in Singapore and Bethlehem, Pennsylvania.
The first sign that the company's systems might have been breached came Monday morning, when email went down. By Tuesday morning, hackers had taken control of all Sands sites, posting what looked like a clip-art collage featuring a map with flames where Sands casinos are located, a snapshot of Sands CEO Sheldon Adelson posing with Israel Prime Minister Benjamin Netanyahu, and a message condemning the use of weapons of mass destruction. The hackers also posted employee Social Security numbers and signed their work, "Anti WMD Team."
Adelson, who is known for his fiery personality, has been outspoken in his support for Israel. In October, he floated the idea of dropping a nuclear bomb on Iran, saying strength was the only thing the country understands. He suggested that the U.S. could begin negotiations over the country's nuclear program by launching a strike on the Iranian desert and threatening to bomb Tehran next.