Microsoft said it was investigating such a motive as well as other possible reasons for the incident, during which a hacker penetrated Microsoft’s computer network and perused blueprints for future products.

Meanwhile, Microsoft president and CEO Steve Ballmer said the hackers had not gained access to any of its key programs or their source code and sought to play down the hack. Asked how damaging the break-in had been, Ballmer said, “Not very, but we want to make sure it doesn’t get that way and that’s why we called in the FBI.’”

The company said engineers first spotted the intruder on Oct.14 and watched as he or she moved through the system until Oct. 25. This suggests the hacker was not, as first thought, inside Microsoft’s system for three months.

The Wall Street Journal, citing sources close to the situation, had reported that hackers were believed to have stolen the blueprints to the latest versions of the company’s flagship Windows OS and Office software suite.

No explanation has yet been given for why someone wanted to hack into the network. Other motives besides industrial espionage range from malice to the challenge of cracking the security systems at the world’s dominant software firm.

However, security experts, and at least one noted hacker disagree. Computer Associates’ vice president of security solutions, Simon Perry, said the sophistication of the operation pointed to an act of industrial espionage. Microsoft said the profit motive was “a possibility, but at this stage we are ruling nothing in and nothing out.”

Perry added that the sophistication of the attack and the targeting of a future product meant it was looking increasingly likely that the intruder was motivated by profit. He said such an intruder might want to auction the source code to the highest bidder. “That bidder might have to be Microsoft. Losing it would affect its competitive position,” he said, noting that firms with competing products might want to acquire the code to steal Microsoft’s best features or improve their own products.

Meanwhile convicted computer hacker Kevin Mitnick said he doubts Microsoft claims that it was watching the intruder that broke into the company’s system.

“It’s hard to tell if you’re getting the straight story,” Mitnick said. “Microsoft claims they were watching the intruder, but Steve Ballmer also said the intruder was looking at source [code]. When somebody breaks into your system, are you going to sit back and [shadow] him or lock him out?”

Mitnick also said he doubted industrial espionage played a role in the attack because of the overall lack of sophistication of the break-in, adding that he finds it unbelievable that the corporate network of Microsoft Corp. would be accessible through static passwords. “That’s very surprising that Microsoft wouldn’t use a higher level of authentication,” he said.

The FBI is thought to be investigating the possibility that information was sent to an e-mail address in St. Petersburg, Russia, but security experts warned that such an address didn’t rule out a hacker much closer to home.