Monday night we heard shouting and thumps outside. The shouts were DC police officers yelling that everyone should get out of a house, the thumps were the officers trying to force the front door with a ram. When we stepped out to look, we could see flames coming out the attic windows. The fire trucks were there in about five minutes, and within a couple more minutes a fireman was up at roof level in a cherry picker spaying water in the east window. Next, another fireman was breaking a hole in the roof with what looked like an ice spade. The visible flame disappeared quickly When we looked out a little later, a fireman had a smaller stream of water playing about ground level on the east side of the house: that proved to be a pile of charred stuff thrown out of windows.
The owners showed up when the last of the crews was stowing the hose to leave. They were there conferring with the police or fire authorities when we went upstairs. Another neighbor says that she could hear saws running in the small hours as somebody cut plywood for the doors and windows.
I had supposed that the police arrived first because of an alarm. Apparently, though, a neighbor heard a window explode, supposed it was a burglar breaking in, and called the police. Another neighbor says that the smell of smoke was very strong when he got home from work, a bit before this; the fire must have had many minutes' start before it showed outside.
The emergency services did well. The police showed up expecting a crime, and quickly turned to trying to alert and evacuate any residents. The fire department arrived in force--four trucks--and quickly put out the fire.