At 08:15, a "Yorktown" SBD piloted by John L. Nielsen sighted Gotō's force screening the invasion convoy. Nielsen, making an error in his coded message, reported the sighting as "two carriers and four heavy cruisers" at , northwest of TF17. Fletcher concluded that the Japanese main carrier force was located and ordered the launch of all available carrier aircraft to attack. By 10:13, the U.S. strike of 93 aircraft—18 Grumman F4F Wildcats, 53 Douglas SBD Dauntless dive bombers, and 22 Douglas TBD Devastator torpedo bombers—was on its way. At 10:19, Nielsen landed and discovered his coding error. Although Gotō's force included the light carrier "Shōhō", Nielsen thought that he saw two cruisers and four destroyers and thus the main fleet. At 10:12, Fletcher received a report of an aircraft carrier, ten transports, and 16 warships south of Nielsen's sighting at . The B-17s actually saw the same thing as Nielsen: "Shōhō", Gotō's cruisers, plus the Port Moresby Invasion Force. Believing that the B-17's sighting was the main Japanese carrier force (which was in fact well to the east), Fletcher directed the airborne strike force towards this target.