Was outside on roof terrace (8th floor) of Hyatt House Downtown Seattle, right next to Space Needle, with 4 friends, preparing for dinner. Mostly clear skies (a few clouds toward horizon, nothing above), just about/after sunset, sky still bluish but fading (getting dark).As I looked toward Space Needle, saw what first seemed like a fast jet coming from the East, heading due West (toward Puget Sound), slightly to the North of us. Not a sound (throughout sighting). Had 2 white blinking lights (pulsating in unison at approximately 1 Hz), as if each wing (L/R) had big white light covering most of wing surface, but could not discern forward fuselage or tail, hence looked like it was a flying straight wing. Still thought it could be airplane despite unusual white lights pattern (am familiar with airplanes; am engineer working in aerospace) and soundless (thought it might be much higher than it seemed); for instance despite white lights flashing, thought they could still be landing lights.Object flew in straight line while displaying white pulsating lights, very fast but at perceived constant linear speed, as it subtended an angle of approximately 45 degrees from my viewpoint and initial spotting. Covered this angle in not more than 10 seconds. Given estimated altitude of twice the height of the Space Needle or more, that would still be very fast for a soundless flight; much too fast for a small drone, or for a helicopter if higher in altitude (yet could not be high enough to be soundless, which would have meant extremely fast).Mentioned helicopter because object came to a complete stop in less than one second, almost right after passing past Space Needle which occulted it for 3-4 seconds. Remained stationary for about 5 seconds, then made in about 2 seconds a lateral translation equivalent to about 1.5 times angle subtended by top circular portion of Space Needle, at 90 degrees from original flightpath, i.e. toward South; stopped again. Very fast acceleration & deceleration in all moves. Then started displaying characteristic red/green position lights pattern and proceeded East in somewhat arced trajectory until losthe from sight within 10-15 seconds. Happened too fast to get colleagues to also witness sightning, when I brought it to their attention colleague nearest to me didn't succeed in spotting it despite my pointing toward it, as it quickly became difficult evendors for me to track. Complete loss of sight by the time it had covered another approximately 30 degrees in direction of Puget Sound.