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'One ought to say,' Phelps said, 'that the data bank being tapped by Sigma was a Defence bank.'

'Yes,' Venn said. 'It was a Defence bank. With two or three taps a week at about ten PM. That was all I knew when I began. However, I made some simple assumptions. First, you've got to have a computer terminal in order to tap the system. That is, once you've called the number that links you to the computer, you must use a teletypewriting or CRT apparatus compatible with the Defence system.'

'Are those terminals common?'

'No,' Venn said. 'They are quite advanced and fairly uncommon. I started with a list of them.' Graves nodded.

'Then I considered the timing. Ten PM Eastern time is seven PM in California, where most of these sophisticated terminals in defence industry applications are located. If an employee were illegally using a terminal to tap into Defence, he couldn't do it during office hours. On the other hand, it requires an extraordinary access to get into an East Coast terminal location at ten at night - or into a Midwest location at eight or nine. Therefore Sigma was probably on the West Coast.'

'So you checked the West Coast terminals?'

'Yes. Because in order to hook into the Defence system, you'd have to unhook from your existing system. What corporation, R&D group, or production unit had a terminal that was unhooked at seven Pm Western time twice a week? Answer: None. New question: What group had its terminals repaired twice a week? Repairing would entail unhooking. Answer: The Southern California Association of Insurance Underwriters, a company based in San Diego.'

Graves said, 'So you investigated the repairman and you found -'

'We found our man,' Venn said, looking slightly annoyed with Graves. 'His name is Timothy Drew. He has been doing repair work on the SC Association computers for about six weeks. It turns out nobody authorized those repairs; he just showed up and -'

'But you haven't picked him up.'

Phelps coughed. 'No, actually. We haven't picked him up yet because he's -'

'Disappeared,' Graves said.

'That's right,' Phelps said. 'How did you know?'

'Tim Drew is a friend of John Wright. He's had dinner with him several times a week for the last month or so.' As he spoke, Graves had a mental image of Drew - early thirties, blond-looking, muscular. Graves had run a check on him some weeks back and had discovered only that Drew was an ex-Army lieutenant, discharged one year before. A clean record in computer work, nothing good, nothing bad.

'We weren't able to find him,' Venn said, 'but we're still looking. We thought -'

Graves said, 'There's only one thing I want to know. What information did Drew tap from the classified files?'

There was a long silence around the cable. Finally Decker said, 'We don't know.'

'You don't know?' Graves lit a cigarette. 'But that's the most important question -'

'Let me explain,' Decker said. 'Drew was an ex-Army officer with knowledge of computer systems. He knew that he couldn't call in on any old number. The call-in numbers are changed at irregular intervals, roughly once a week. But the possible permutations of the call-in number aren't great. With trial and error, he might have found it.'

'You know he found the number,' Graves said, 'because you know he tapped in. The question is, what did he tap out from the system?'

'Well, once he was hooked up, he still had a problem. You need subroutine codes to extract various kinds of information, and -'

'How often are the codes changed?'

'Not very often.'

Graves found himself getting impatient. 'How often are the codes changed?'

'About once a year.'

Graves sighed. 'So Drew might have used his old codes to get what he wanted?'

'Yes.'

'Then we want to know what codes he knew. What sort of work did Drew do when he was in the Army?'

'He did topological work. Surface configurations, shipment routings, that sort of thing.'

Graves glanced at Phelps. 'Can we be more specific?'

'I'm afraid not,'