China has rejected accusations it was behind an attack that stole data from health insurer Anthem.
Personal details of up to 80 million people are believed to have been stolen in the hack.
Investigators looking into the breach told US newspapers the methods used in it resembled earlier attacks blamed on China.
China said it was "careless" to blame it when the origins of hack attacks were hard to pinpoint.
On Thursday, Anthem - America's second largest health insurer -revealed that it had been the victim of a "very sophisticated external cyber-attack" in which data about millions of its customers had been stolen.
If the figure of about 80 million people is confirmed, it will be the biggest theft of medical-related customer data.
Investigators looking into the breach told Bloomberg that early indications suggest China was behind it. They said the attack had the "fingerprints" of a nation-state and some of the techniques used were seen in earlier attacks that had been blamed on China.'No evidence'
The fact that many of those insured by the company work for government contractors or in sensitive industries has led investigators to speculate it was done to gather information about potential targets rather than for financial gain.
Bloomberg said another health insurer in the US had been hit in the same way as Anthem and an investigation into that attack revealed its aim was to gather data about defence contractors working on advanced avionics and weapons systems.
In response, China said accusations against it by the US were "groundless".
"It is unreasonable to make an accusation without enough evidence," a foreign ministry spokesman said during a briefing in Beijing.
"It is very difficult to determine the source of hacking activities, especially when it is carried out across borders," said the spokesman.
The scale of the attack has now led attorneys general from five states to start their own investigations. In addition, California's Department of Insurance is assessing how Anthem has responded to the attack.
Today, senior staff from Anthem are scheduled to tell the Energy and Commerce Committee of the US House of Representatives about the hack attack and how it is being handled.
Michael Daniel, who was recently appointed to be President Obama's cybersecurity adviser, said the attack was "quite concerning".
"It's particularly disturbing especially when it hits that many people," he said and advised people to change passwords and keep an eye on their credit ratings.