By then, Northwest Flight 188 with its 144 passengers and five crew members was over Wisconsin, at 37,000 feet.
The pilots - Richard Cole of Salem, Ore., the first officer, and Timothy Cheney of Gig Harbor, Wash., the captain - denied they had fallen asleep as aviation experts have suggested, the safety board said in recounting investigators' interviews with the men over the weekend.
Instead, Cole and Cheney said they both had their laptops out while the first officer, who had more experience with scheduling, instructed the captain on monthly flight crew scheduling.
A number of aviation experts - and people wondering about their next airline flights - have been suggesting it was more plausible that the pilots had fallen asleep during the San Diego-to-Minneapolis flight than that they had become so focused on a conversation that they lost awareness of their surroundings for such a lengthy period of time.
Air traffic controllers in Denver and Minneapolis repeatedly tried without success to raise the pilots by radio. Other pilots in the vicinity tried reaching the plane on other radio frequencies. Their airline tried contacting them using a radio text message that chimes.
Authorities became so alarmed that National Guard jets were readied for takeoff at two locations and the White House Situation Room alerted senior officials, who monitored the airliner as the Airbus A320 flew across a broad swath of the mid-continent out of contact with anyone on the ground.
"It's inexcusable," said former NTSB Chairman Jim Hall. "I feel sorry for the individuals involved, but this was certainly not an innocuous event - this was a significant breach of aviation safety and aviation security."
Delta said in a statement that using laptops or engaging in activity unrelated to the pilots' command of the aircraft during flight is strictly against the airline's flight deck policies. The airline said violations of that policy will result in termination.
There are no federal rules that specifically ban pilots' use of laptops or other personal electronic devices as long as the plane is flying above 10,000 feet, said Diane Spitaliere, a Federal Aviation Administration spokeswoman.
"I think it depends upon how it's being used," Spitaliere said.
The Air Transport Association, a trade group that represents major U.S. airlines, expects pilots to comply with federal regulations and airline policies, but hasn't taken a position on the use of electronic devices by pilots while in the cockpit, ATA spokeswoman Elizabeth Merida said.
Delta has suspended the two pilots pending an investigation into the incident. FAA is also investigating and has warned Cheney and Cole their pilot licenses could be suspended or revoked.
Pilots' schedules are tied to their seniority, which also determines the aircraft they fly. Those at the top of the list get first choice on vacations, the best routes and the bigger planes that they get paid more for flying. Following Delta Air Lines' acquisition of Northwest, an arbitration panel ruled that the pilot seniority lists at the two carriers should be integrated based on pilots' status and aircraft category.
Cheney and Cole are both experienced pilots, according to the NTSB. Cheney, 53, was hired by Northwest in 1985 and has about 20,000 hours of flying time, about half of which was in the A320. Cole, 54, had about 11,000 hours of flight time, including 5,000 hours in the A320.
Both pilots told the board they had never had an accident, incident or violation, the board said.
The pilots acknowledged that while they were engaged in working on their laptops they weren't paying attention to radio traffic, messages from their airline or their cockpit instruments, the board said. That's contrary to one of the fundamentals of commercial piloting, which is to keep attention focused on monitoring messages from controllers and watching flight displays in the cockpit.
"It is unsettling when you see experienced pilots who were not professional in flying this flight," said Kitty Higgins, a former NTSB board member. "This is clearly a wakeup call for everybody."
Sen. Amy Klobuchar, D-Minn., called the incident "the ultimate case of distracted driving, only this time it was distracted flying."
AP Airlines Writer Harry R. Weber contributed to this report from Atlanta.