SAN JOSE, Calif. - Rajeev Motwani, 47, the Stanford University computer-science professor who mentored Google's founders when they were graduate students, died in a freak accident at his Atherton, Calif., home.

Mr. Motwani apparently drowned early Friday morning in a backyard swimming pool at the home he bought three years ago. Friends said he did not know how to swim, but was planning to take lessons.

While little official information was available, friends speculated that Motwani, who sometimes stayed up late reading and answering e-mail, may have stepped outside for some fresh air, become disoriented in the dark, and fallen into the pool.

He is survived by his wife, Asha Jadeja, and daughters, Naitri and Anya.

The news of his death struck like a thunderclap in Silicon Valley.

Blogs, Facebook pages, and Tweeter communications were filled with testimonials to a brilliant, kind man who was never too busy to help a budding entrepreneur or struggling graduate student.

He assisted many Silicon Valley start-ups gain a foothold, but none so famous as Google, whose cofounder Sergey Brin mourned him Saturday.

"Today, whenever you use a piece of technology, there is a good chance a little bit of Rajeev Motwani is behind it," Brin wrote Friday at http://too.blogspot.com/2009/06/remembering-rajeev.html

"We've lost one of the best contributors to the Silicon Valley ecosystem," said a friend, angel investor Ron Conway.

"Everybody in Silicon Valley talked to him," said David Hornik, a venture capitalist. "Rajeev was an incredible resource to everyone."

Born on March 26, 1962, in Jammu, India, Motwani grew up in New Delhi, earned a computer-science degree from the Indian Institute of Technology - Kanpur in 1983 and his doctorate from the University of California at Berkeley, in 1988.

As a Stanford professor, he also was the director of graduate studies for the Computer Science Department, and founded the Mining Data at Stanford project.

His work had a major effect on the field of algorithms, and he used his knowledge of that field to develop methods for searching almost infinite archives of data by randomly selecting subsets of the data.

Data mining is the basis of much of modern Internet commerce and the operation of search engines such as Google.

Mr. Motwani was awarded the Gödel Prize for his contributions to theoretical computer science, and he also earned an Alfred P. Sloan Research Fellowship.

His research ranged over the computer-science fields of search and information retrieval, streaming databases, data mining, robotics, and even drug design.

Though he was not Brin's official adviser, the Google cofounder wrote in his blog that "he played just as big a role in my research, education, and professional development. In addition to being a brilliant computer scientist, Rajeev was a very kind and amicable person and his door was always open."

A paper published in 1998 by Google cofounders Brin and Larry Page, Mr. Motwani and Stanford computer-science professor Terry Winograd, "What You Can Do With A Web In Your Pocket," described the development of a global ranking of Web pages called PageRank and "a novel search engine called Google."