The difference is a big one. The United States has already indicted five members of a People’s Liberation Army unit that is thought to be responsible for stealing intellectual property, usually designs from American and European companies. But the group is not known for stealing large amounts of personal information.

The key is the hackers’ motive. While they could be preparing to sell the information on the black market, they may also be searching for intelligence on government officials or senior executives who mask their personal information, but tend to provide real names and real numbers when dealing with health-related matters.

“The question is whether this is about espionage or theft,” said one government official who was briefed on the investigation.

As a rich source of personal information, health care organizations — like hospitals, doctors’ offices and insurers — are increasingly going to be vulnerable to attacks, according to security experts. “Anthem is not the last of these organizations,” said Cameron Camp, a researcher for ESET, which specializes in data security. “We’re going to see that style of attack again.”

Medical identify theft is on the rise, experts say, because it pays. In black-market auctions, complete patient medical records tend to fetch higher prices than credit card numbers. One security expert said that at one auction a patient medical record sold for $251, while credit card records were selling for 33 cents.

After the large data breaches at major retailers like Target and Home Depot, the black market for credit cards has been flooded. And after the bank becomes aware of the theft, those cards are usually canceled quickly.

In contrast, patient medical records typically include information not easily destroyed, including date of birth, Social Security numbers and even physical characteristics that make them more useful for things like identity theft, creation of visas or insurance fraud by falsely billing for expensive medical or dental procedures that were either never done or performed on someone else. Some criminals have also tried a form of so-called ransom ware in which they threaten to reveal medical information unless they are paid.