NEW YORK (Reuters) — The top federal prosecutor in northern California said yesterday that the work of a US task force probing stock options backdating at companies in his area could go on for “perhaps years to come”.Kevin Ryan, the US attorney for the Northern District of California, declined to comment on whether his office was about to bring new criminal charges related to the options probes. The task force was formed in July to probe possible wrongdoing by San Francisco Bay Area companies and individuals related to the timing of stock options awards.

“We think we will continue to be engaged for perhaps years to come while we work these cases out,” he said at a legal conference in Manhattan. The task force includes officials from the US Justice Department, the FBI as well as the Internal Revenue Service.

The team is looking at stock options award practices at about 25 companies, Ryan told Reuters after his speech. Silicon Valley technology companies, which used option grants as employee recruitment and retainment tools during the 1990s bull market, are a big focus of the probe.

Ryan said it could take several years to carry out the investigations because there are so many people to interview and documents to sort through, but the task force also is “mindful of moving them (the cases) along.”

More than 160 US<\p>companies have launched internal investigations or are the subject of regulatory probes to determine if the dates of stock options were manipulated. More than 50 executives have resigned.

The probes typically centre on whether companies awarded executives backdated options after a spike in the share price, making the options more valuable.