It's described beginning on page 43 of this doc.
Antonio Rullan sent me the complete case file, and I have to say it almost competes with the RB-47 case as far as quality. It's a multi-radar/visual/aircraft incident, totally military, and went to White House levels. Apparently LBJ or one of his national security staff was notified about it, and the WH demand for answers caused some turmoil.
The object resembled an SR-71 (or predecessor Oxcart) but I sent the file to somebody who was working on a history of those planes and he could find no reference to them being operated in the Puerto Rico area so early in their development. Besides, it was checked at the time. Also, the UFO performance was too high even for those planes.
It's a very complex and interesting case, particularly in terms of the official reaction not to mention the incident itself.