After everyone had left, Jack Mattera entered a fourth-floor office in Manhattan's financial district about 8 p.m. one day last month. The company president led him to the computer of an employee suspected of selling company secrets.

Mattera took two hours to copy its hard drive onto a special piece of recording equipment he brought with him, without leaving a trace.

Working in a computer forensics lab in Pennsauken the next day, Mattera found letters and appointment schedules on the copied hard drive that immediately confirmed that the employee had handed over pricing and other proprietary information to a competing firm.

The employee was fired within days.

More and more, corporations are hiring computer forensics experts such as Mattera to search for hidden or deleted computer files that might contain evidence of wrongdoing in the workplace. In other instances, these experts are asked to retrieve accidentally lost or deleted material.

However a file was erased, these experts can usually reconstruct most, if not all, of the text.

"There is data you can see, and data I can see," Mattera said. "When you delete a file, it doesn't go away."

A former investigator for the U.S. State and Treasury Departments, Mattera directs computer investigations at the Intelligence Group, a corporate investigation firm based in Far Hills, N.J. He also oversees its Philadelphia sales office. The company, which was founded in July 1999, expects revenue to rise from $1.5 million last year to $2 million this year, Mattera, 42, said.

With a full-time staff of 20, the Intelligence Group competes with larger computer-security firms such as Kroll Inc., of New York; Computer Evidence Discovery Inc., of Seattle; and a growing number of one- and two-person forensics operations, often run by retired law enforcement officers. Large accounting firms are also getting into the business.

To help handle the growing number of cases involving computers, Mattera launched the company's forensics lab two months ago in a nondescript office suite equipped with six computers.

Most of his office's work involves cases of alleged wrongdoing. He said that, when he does uncover such evidence, he sometimes advises the company that hired him to contact law enforcement officials.

"There isn't a crime today that doesn't involve a computer," Mattera said. "It's no different in the corporate world."

A job can run a client anywhere from $200 to $450 an hour.

The Intelligence Group might be asked to verify the history of an electronic file, including the dates on which it was created and modified. Other assignments have included searching through months or years worth of e-mails for messages relevant to a case, tracking the source of anonymous e-mails or postings, investigating computer intrusions, or simply determining whether an employee is viewing pornographic files at work. The firm also reviews companies' policies on computer security and privacy, and makes recommendations.

Mattera said that, because many computer files might become court evidence, his staff was trained to retrieve data without altering or losing even one byte that could compromise the case.

When a lawsuit arises, attorneys who once needed to worry about following only the paper trail now must depend on computer forensics experts for the monumental task of what they call "e-discovery."

A recent study by the University of California-Berkeley said that 93 percent of all new information is created electronically.

"Every lawyer knows, if you're not going after e-mails, you're completely missing the boat," said William Gyves, a partner at Entwistle & Cappucci L.L.P., of New York. "Unless you have the capacity inside your firm, you absolutely have to be in touch with a forensics expert in litigation of any complexity."

What many computer users don't realize is that neither emptying a computer's recycle bin nor reformatting the hard drive will erase the multiple versions of a file that continue to exist, tucked away in the machine's far corners until they are overwritten by new material. That could take months or even years, Mattera said.

For one client, Mattera searched the computer of an employee accused of fabricating checks in the names of coworkers. He learned that the man had visited Web sites that sold phony copies of checks, fake driver's licenses, and other identification.

Using special software, such as EnCase or Forensics Toolkit, Mattera said he turned up relevant files quickly by conducting keyword searches of every bit on the drive for the names of specific projects or people.

"Nobody ever thinks someone is going to look at their stuff," said Mattera, who has found e-mails of employees discussing how they might steal company information, and whether they could go to jail for it.

Even when Mattera fails to find proof of wrongdoing, he might uncover other valuable information that a company might deem important to protecting its business interests.

One time, he found that workers who left one company to join a competing firm were romantically involved with employees at the original business. The employer "wouldn't want these people to be assigned to sensitive information," Mattera said.

Some of what computer investigators look into might raise privacy issues, but employees don't have a leg to stand on because few laws protect workers from electronic surveillance, said Lewis Maltby, president of the National Workrights Institute in Princeton.

More than three-quarters of large U.S. companies conduct some form of electronic surveillance of their employees, according to a study by the American Management Association in New York.

"Employers aren't even legally obligated to tell employees about the monitoring," Maltby said. "An employer has the legal right to conduct monitoring and keep the entire program secret from employees."

To do his job, Mattera said he combined police instincts and training with his expertise in cyber security.

Mattera worked as a North Wildwood police officer in his mid-20s. From 1985 to 1987, he worked in the State Department investigating passport and visa fraud. He also served as a bodyguard for Secretary of State George P. Shultz on a number of his overseas trips.

Other times, he served in the same role in this country for such visiting dignitaries as Princess Diana, Prince Charles, Israeli Foreign Minister Shimon Peres.

Later, while working in the Inspection Service of the Internal Revenue Service, Mattera said he made the transition to computer investigations, and eventually became one of four computer investigative specialists for the division.

Mattera "looks more like a cop than a computer science professor, and that's helpful because he can relate to clients," said Gyves, the Entwistle & Cappucci partner. "Sometimes you need that at the very beginning of a case to find out where the bodies are buried."