The SECRET HQ AAF letter dated 26 September 1947 states, "Speed: In excess of 500 mph (435 knots). Altitude: 9000 ft. or higher".
July 1, 1947; GCA Unit, Chitose, Hokkaido, Japan
A target was picked up 16 miles north of the base on a course of 180 degrees. The GCA unit immediately called the tower and asked if any aircraft were operating north of the field. At this instant of this radio transmission, the target reversed course to 360 degrees and was tracked out to a range of 28 miles. The target then changed course to 240 degrees arid was tracked out 6 miles where it again reversed course to 060 degrees and returned to the original point 28 miles north of the base. The target then changed course to 360 degrees and faded out of range. The nearest known operational U.S.S.R. air base to the north of Chitose AAB is Otomari on southern Ostrov Sakhalin at a range of 234 nautical miles from Chitose AAB. This base is classified as having temporary runways of 4500 - 6000 foot length. Base contains a large aviation gasoline tank farm with a pipe line to Korsakov Harbor which is capable of handling tankers. Headquarters of the U.S.S.R. Air Force on Karafuto is located in the Toyohara area about 24 miles to the north of Otomari. 

Detailed reports and documents
Item "a" from this three-page SECRET letter from HQ AAF (Fran Ridge/Jan Aldrich)
RADCAT Listing (Martin Shough/Jan Aldrich)