Lawmakers in the U.S. asked Japanese firm Sony Corp. for additional information about an attack that exposed more than 100 million customer accounts to hackers, an event its chief executive officer described as a 'hiccup.'

Representatives Mary Bono Mack, a California Republican, and G.K. Butterfield, a North Carolina Democrat, sent a letter to Tokyo-based firm Sony requesting details on what customer information may have been stolen and whether the company's investigation has determined how the breach occurred.

The letter, addressed to Kazuo Hirai, Sony's executive deputy president in charge of consumer products and network services, requests a response by May 25.

Sony declined to appear at a May 4 hearing on data theft chaired by Ms Bono Mack and instead sent an eight-page letter responding to lawmakers' questions about the attack that crippled the company's PlayStation gaming network.

Ms Bono Mack plans to hold a follow-up hearing and introduce legislation on data security, her spokesman Mr Ken Johnson said in an email quoted by Bloomberg News.

"Clearly, there are still a lot of unanswered questions," Mr Johnson said. "As we begin drafting our data security legislation, we are hopeful that the Sony experience can be instructive."

The hacker attack, which forced Sony to shut down its online movie, music and games services, is a "hiccup" in the company's online strategy, Chairman and Chief Executive Officer Howard Stringer said in an interview yesterday, reports from Bloomberg said.