Thousands of jumbo flying squid, aggressive 5-foot-long sea monsters with razor-sharp beaks and toothy tentacles, have invaded the shallow waters off San Diego, spooking scuba divers and washing up dead on beaches. The so-called Humboldt squid, which can grow up to 100 pounds, are native to the deep waters off Mexico, where they have been known to attack humans. Scientists are not sure why the squid are swarming off the Southern California coast, but they are concerned. In recent years, small numbers have been spotted from California to Sitka, Alaska, an alarming trend, scientists say. In 2005, a similar invasion off San Diego delighted fishermen and, in 2002, thousands of squid washed up on the beaches. This summer, the wayward squid have also been hauled up by fishermen in waters off Orange County, just north of San Diego. Research suggests the squid may have established a year-round population off California at depths of 300 to 650 feet, said Nigella Hillgarth, executive director of the Birch Aquarium at Scripps Institution of Oceanography. The swarms may occur when their prey moves to shallow waters and the squid follow, Ms. Hillgarth said.