It seemed to be one more bit of damning evidence on a day full of American military mistakes.

On the morning of Dec. 7, 1941, two Army signals soldiers operating a mobile radar at Opana station, 532 feet above sea level near Kahuku Point, saw the biggest blip they had ever seen on their oscilloscope.

This was, in fact, the first wave of 183 invading Japanese planes, although all the operators knew was it appeared to be a large group of aircraft.

The response given by Lt. Kermit Tyler at Fort Shafter when it was called in — “Don’t worry about it” — has gained its own infamy.

The utterance had as much to do with American military complacency on Oahu as missing a forewarning of the attack.

In September 1941 five mobile radar sets began operating. The sixth, at Opana, amounting to three trucks and a 45-foot antenna, was added Nov. 27. At that point they were being operated solely for training, the U.S. Army notes.

Lt. Gen. Walter C. Short, head of the Army’s Hawaiian Department, was unconvinced of the value of radar and didn’t understand it, wrote Thurston Clarke in “Pearl Harbor Ghosts.”

“This attitude explains why radar stations on Oahu only operated between 0400 and 0700 on Sundays, and sporadically during the week, why the Army failed to establish approach lanes enabling friendly aircraft to be separated from enemy ones, and why the Hawaiian Department’s fighters were on a four-hour alert, meaning it would take that long to arm and scramble them,” Clarke said.

Pvt. George E. Elliott Jr., who operated the radar along with the more experienced Pvt. Joseph Lockard, later estimated the planes were 137 miles out to sea.

Tyler, a fighter pilot, was on duty at Fort Shafter’s Information Center along with seven or eight enlisted men. At 7 a.m. most went off duty, leaving only Tyler and the switchboard operator.

Tyler later told the Navy court of inquiry that it was his second day on the job, that he was there chiefly for training and was given no specific duties other than to report for duty.

He said he had “very good reason to believe” that a flight of B-17 bombers was en route to the islands from the mainland. In fact, a dozen were on their way at the same time as the Japanese attackers.

“I had a friend who was in the bomber command who told me that any time the radio stations were playing this Hawaiian music all night, I could be certain that a flight of our bombers was coming over,” Tyler told the board. “When I had gotten up at 4 a.m. to report for duty, I listened to this music all the way to the station, so I was looking for a flight of B-17s.”

The Army had ordered radio station KGMB to broadcast all night to provide a beacon for the U.S. bombers. The Japanese planes used it, too.

Tyler said he deduced that the incoming planes spotted by Elliott and Lockard were B-17s or U.S. Navy planes. He was not on alert for enemy planes. Rather, it was “just the opposite, because we had been on alert about a week before, and the alert had been called off.”

Tyler was not disciplined for his actions. The Navy Court of inquiry in 1944 found that the radar warning system “was ineffective on the morning of Dec. 7” in part because there was no way to distinguish friendly from enemy planes.

Gordon W. Prange, Donald M. Goldstein and Katherine V. Dillon said in “At Dawn We Slept” that there was a grievous misjudgment.

“Neither at the time nor later in the day did anyone in the Army outfit notify the Navy of the Opana sighting,” the authors wrote. “This was a serious error because this clear track would at least have revealed the direction of the Japanese carriers and saved the Navy’s later searchers a long, weary wild goose chase.”