John Markoff writes today on security expert Tsutomu Shimomura, the target of the break-in Markoff wrote about on Monday. For email copy send blank message with subject: TSU_shi Here's an excerpt: Mr. Shimomura, one of the country's most skilled computer security experts, is the person who prompted a Government computer agency to issue a chilling warning on Monday. Unknown intruders, the agency warned, had used a sophisticated break-in technique to steal files from Mr. Shimomura's own well-guarded computer in his home near San Diego. And the stealth and style of the attack indicated that many of the millions of computers connected to the global Internet network could be at risk. There have been at least four other known victims so far, including computers at Loyola University of Chicago, the University of Rochester and Drexel University in Philadelphia. Since Monday, as the F.B.I. has continued to investigate the crime and look for evidence of break-ins elsewhere, Mr. Shimomura has been answering telephone calls and E-mail from government, corporate and university computer administrators seeking advice on how to arm themselves. Between replies, he has been working feverishly to perfect a new type of protective software that would thwart the burglars. Once it is finished, he intends to distribute the software free over the Internet. But more than anything else, Mr. Shimomura, who is 30, wants to help the Government catch the crooks. And while he acknowledges that the thieves were clever, Mr. Shimomura has also uncovered signs of ineptitude that he says will be the intruders' eventual undoing. "Looks like the ankle-biters have learned to read technical manuals," Mr. Shimomura said derisively. "Somebody should teach them some manners. "