As Benjamin Shayne settled into his back yard to listen to the Orioles game on the radio Saturday night, he noticed a small plane looping low and tight over West Baltimore — almost exactly above where rioting had erupted several days earlier, in the aftermath of the death of a black man, Freddie Gray, in police custody.

The plane appeared to be a small Cessna, but little else was clear. The sun had already set, making traditional visual surveillance difficult. So, perplexed, Shayne tweeted: "Anyone know who has been flying the light plane in circles above the city for the last few nights?"

That was 9:14 p.m. Seven minutes later came a startling reply. One of Shayne's nearly 600 followers tweeted back a screen shot of the Cessna 182T's exact flight path and also the registered owner of the plane: NG Research, based in Bristow, Va.

"The Internet," Shayne, 39, told his wife, "is an amazing thing."

What Shayne's online rumination helped unveil was a previously secret, multi-day campaign of overhead surveillance by city and federal authorities during a period of historic political protest and unrest.