A disgruntled computer engineer has been sitting in a prison cell for four days after locking out everyone but himself from a city computer system.

Terry Childs is accused of tampering with San Francisco's new computer network to give himself exclusive access. He has refused to hand over the password and is being held on $5 million bail while officials try to crack his code.

The Department of Technology employee, 43, allegedly created a secret password to the city government's data network. The multimillion-dollar network stores records such as officials' e-mails, city payroll files, confidential law enforcement documents and jail bookings.

Mr Childs was arrested last Sunday and is being held on suspicion of four counts of computer tampering. His arraignment is scheduled for later today.

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The city says that fixing the system, which hosts 60 per cent of the city government's data, and determining whether the alleged tampering led to a security breach could cost millions of dollars.

At a news conference announcing Mr Childs' arrest, Kamala Harris, the district attorney, said: "This involves compromising a public system that we rely on. Its integrity has been compromised."

The network administrator, who lives in Pittsburg, has been working for the Department of Technology for five years and has a basic salary of just over $126,000. Police said that Mr Childs had recently been disciplined at work. Reports suggest that officials then discovered that Mr Childs had put together a tracing system to monitor what other administrators were saying and doing in relation to his disciplinary case.

Last weekend he allegedly locked out all administrators except himself. Before his arrest he gave pass codes to police, but they did not work. "They weren't able to do it," an official said. "This was kind of his insurance policy."

It was feared that although Mr Childs is in jail, he may have enabled someone else to access the system by telephone or other electronic device and order the destruction of hundreds of thousands of sensitive documents.

Currently documents in the system can be accessed, but if there are problems or the system crashes there is no way to repair the network quickly. Engineers from the technology company Cisco have been brought in to try to gain access.

Gavin Newsom, the mayor of San Francisco told the San Francisco Chronicle: "There is nothing to be alarmed about, save the inability to get into and tweak the system. Nothing dramatic has changed in terms of our ability to govern the city."

He said that Mr Childs had been a highly regarded member of staff but was now a "rogue employee that got a bit maniacal".

It has emerged that Mr Childs has a criminal record for aggravated burglary, for which he served five years' probation in 1982.

"He was very good at what he did, and sometimes that goes to people's heads. We think that is what this is about," Mr Newsom said.

The incident is one of the highest profile cases of what IT security experts call "insider threat". Layers of security and checks are built into companies' computer system to protect against disgruntled and unscrupulous employees trying to gain access to valuable information and sharing it for personal gain, espionage or revenge.

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