The day after Sony Pictures employees discovered that company email was unusable following a cyberattack, senior executives came up with an old-style communication network: a phone tree, in which updates on the hack were relayed from person to person.

With computers down during Thanksgiving week, the Sony Corp. 6758, +2.34% studio’s 6,000 employees were forced to improvise, with cellphones, Gmail accounts and notepads. The payroll department dug up an old machine to cut paychecks manually. Before long, the studio unearthed a cache of BlackBerrys, which still worked because they send and receive email via their own servers.

Sony Entertainment Chief Executive Michael Lynton told a meeting of senior executives that hackers hadn’t simply stolen data. They had erased it, rendering the entire computer system unusable.

“It took me 24 or 36 hours to fully understand this was not something we were going to be able to recover from in the next week or two,” Lynton recalled in an interview.

The next several weeks would make clear that Sony’s film and television studio was the victim of one of the most malicious cyberattacks in history — one that would result in the leak of hundreds of thousands of sensitive documents and embarrassing emails, the worsening of tensions between the U.S. and North Korea, a flip-flop on the release of a politically sensitive movie and damage to the company’s relationships with stars and theater owners.

The cascade of crises threw a spotlight onto the analytical and reserved Lynton, who has long had a lower profile in Hollywood than his top deputy, motion-picture chief Amy Pascal, who works out of a larger office suite than he. Lynton, a 54-year-old former publishing, movie and Internet executive, has run Sony Pictures since 2004, but was known as a largely hands-off manager until the past month.

While the studio has been closed for the holidays, Lynton has been personally pursuing deals for wider distribution of “The Interview” in theaters and online, negotiations that normally fall to executives several levels beneath him. “I have tried to make sure all the decision-making related to this incident comes back to me so that, as much as possible, the operating groups are not distracted from the normal business they have to do,” he said.

Over Thanksgiving weekend, the IT department scrambled to get basic systems like email back online. Agents from the Federal Bureau of Investigation worked nearby, as did investigators from FireEye Inc., a cybersecurity company that deploys Ghostbusters-like teams to companies that have been hacked.

Kevin Mandia, FireEye’s chief operating officer, called the confluence of stolen credentials, erased hard drives, and leaked documents at Sony unprecedented in the history of corporate cyberhacks.

Though the hackers ordered Sony in their initial message to “obey us,” they never identified themselves or issued demands. Instead, they created maximum chaos. The week after the hack, the perpetrators leaked five Sony movies onto the Internet, along with thousands of internal documents and the Social Security numbers and other personal information of more than 47,000 people, including current and former employees, freelancers and a handful of movie stars.

FireEye’s investigators searched for clues on who had broken into the systems and when. But so much data had been destroyed that they have had trouble retracing the hackers’ steps. They still can’t confirm that the hackers have been eradicated from Sony’s systems, two people familiar with the investigation said.

An expanded version of this report appears on WSJ.com