When pioneer computer scientist James Gray went missing last week, his friends and colleagues quickly launched a high-tech, grassroots search effort that reflected the many fields of research and business Gray, 63, had touched and, in some cases, they relied on tools he had helped to develop.

Gray, a researcher and manager of Microsoft Research's eScience Group, was reported missing on January 28 after failing to return to his San Francisco home from a sailing trip to the Farallon Islands, 30 miles west of the Golden Gate Bridge, where he had headed that morning to scatter his late mother's ashes. The U.S. Coast Guard, notified by Gray's wife, immediately mounted a search up and down the California coast that would extend more than 300 miles offshore and cover 132,000 square miles. But they called off the effort four days later after failing to turn up a single trace of Gray or his 40-foot C&C sailboat Tenacious.

After the first day of searching turned up nothing, Gray's colleagues prepared to join in the hunt the way they knew best: electronically. "Jim [is] tremendously well liked in the industry," says Mike Olson of Oracle Corporation, spokesperson for the search effort. "It took very little to bring a lot of people into the group." By Wednesday, January 31, after numerous requests, a flyby was arranged from satellite-imaging company Digital Globe, which supplies images for Google Earth.

Chuck Herring, a Digital Globe spokesperson, says the company's satellite was in a position to view the area on Thursday and again that weekend, so officials redirected its camera to canvas strips of then partly cloudy coast in 11-by-11-mile chunks. NASA researchers also arranged to have a high-altitude aircraft, the ER-2, take photos of the region.

Amazon.com employees, Johns Hopkins University researchers and others sorted through the images for clues. Gray had worked for 10 years with Hopkins scientists to implement the Sloan Digital Sky Survey, a massive, computer-driven effort to piece together a picture of space one snapshot at a time. "He literally changed the way we do astronomy today," says Alex Szalay, a Hopkins astrophysicist and longtime Gray collaborator who helped organize the sorting effort.

Gray had also created the tools that made it possible for Amazon.com to exist, by inventing the method for secure electronic transactions that underlies ATM withdrawals and online credit card purchases, among other things, says Amazon's chief technology officer Werner Vogels, another coordinator of the image sorting.

Once the aerial images were available to download, the image processing teams began breaking them into smaller pieces suitable for quick visual inspection. Herring says the satellite images have a resolution of 0.6 meter (two feet), enough to make out a boat.

To swiftly identify potential hits from the hundreds of thousands of resulting images, they were fed into an Amazon system called Mechanical Turk, originally designed to expedite tasks that are hard for computers (such as distinguishing adult content from other images). The Amazon team displayed the images online for volunteers to help sort.

By Tuesday evening on February 6, roughly 12,000 sorters had run through all 560,000 satellite and ER-2 images, according to a post by Vogels on the Tenacious Search blog, which was established to coordinate the search effort.

Searchers also flew up and down the coast in private planes and plastered the coast with "missing" posters of Gray.

"It's pretty rare" for there to be no physical evidence in a boating incident, says Lt. Amy Marrs, spokesperson for the Coast Guard's San Francisco sector. "If there's a collision or a boat sinking, something is going to float." She notes that the weather for sailing was excellent the day Gray disappeared blue sky, a gentle breeze, calm water.

Olson says image analysts now have to sift through possible man-made objects in flagged images and combine that information with models of coastal drift. Olson says Gray himself had worked on methods to calculate wind and drift direction from buoys and other sensor data: "He collaborated with people on satellite image data capture and analysis and sea surface data capture and analysis, and these people now are using their professional skills in the search."

"We understand that the Coast Guard are the pros at searching," he adds. "Our goal is to produce likely targets for putting up more planes and more boats for targeting specific areas of the ocean."

Marrs says the Coast Guard has suspended Gray's case indefinitely pending further leads. "If a credible lead comes up then we will reopen the case and go back out," she says.

Marrs calls the search effort "very impressive," noting that it is "the largest strictly privately sponsored search that we've ever seen." She would not speculate on Gray's fate, but she did say that cases lasting for more that a few days are rarely resolved. "When a case is suspended," she says, "it's not often that ... we find a final answer on it."

Friends and colleagues describe Gray as a careful, methodical veteran sailor, who for a few years had even lived on a boat. He did not seem distraught or unusually upset by his mother's death, Vogels says, noting that, if anything, Gray appeared relieved that his elderly mother was no longer suffering from a protracted illness, and he was in good spirits after becoming a grandfather last year.