Sony Corp. was warned about a year ago that hackers had infiltrated its network and were stealing gigabytes of data several times a week, underscoring a pattern of lapses predating the recent attack that has spilled Sony Pictures' secrets onto the Internet.
The hackers, who have not been identified, sifted in late 2013 through data from the company's network, encrypted the information to cover their tracks and mined it on a regular schedule, said a person familiar with Sony's investigation of the breach who asked not to be named because the findings are confidential.
The company's cybersecurity problems date at least as far back as 2011, with a breach of Sony's PlayStation video-game network.