Al Qaeda and other terror groups are more sophisticated in their use of computers but still are unable to mount crippling Internet-based attacks against U.S. power grids, airports and other targets, the FBI's top cyber crime official said Wednesday.

Investigators keep a close watch on terror groups' use of computers but have not detected any plans to launch cyber attacks against major public institutions in the United States, FBI assistant director Louis M. Reigel said.

"I don't think that capability exists today," Reigel said in a briefing for reporters at FBI headquarters.

The government has conducted simulated terrorist attacks on computer, banking and utility systems, and Reigel said his division of around 1,100 agents treats seriously the prospect of such a strike.

FBI cyber experts have noticed progress in the technical mastery suspected terrorists have shown online, he said. One new wrinkle first appeared four months ago, Reigel said, declining to be more specific.

Terrorists also have made only infrequent use of stenography, the practice of hiding a text message in another kind of file, typically a picture, Reigel said. "It looks like a picture, but if you have the right program, you can extract a text message embedded in a picture," said Reigel, a 31-year FBI veteran who formerly led the New Orleans field office.

On another matter, the FBI still has no suspect in the spread of the latest version of a computer worm that began appearing last month as e-mails purporting to come from the FBI, CIA and German security services, Reigel said.

The third version of the Sober worm spread so quickly and widely that at one point the FBI was bombarded with 200,000 e-mails a minute over four days, he said. "It almost killed our system," Reigel said, before technicians developed a means to divert the messages.

Unlike with earlier versions, "this time we believe we have enough information to where we can pursue a logical investigation," he said.