It happened on Fourth Street between 6th and 7th Avenues on Monday morning. Hepcat saw this on Gothamist this evening. I was out of the Slope all day. So I missed this:

Gothamist received an email from a 4th Street resident, there was a DEP/HAZMAT truck/mobile unit parked on the block. The resident called the 78th precinct on her block:

“An unidentified liquid has been removed from a residence. Nothing to be worried about.” The officer declined to elaborate further, and the street is currently blocked off.

An update on Gothamist says:

According to the NYPD, the incident in Park Slope this morning involved one fatality: 65-year-old Robert Siegel, whose body was found by his wife. The DEP tested the toxic liquid and determined the substance to be cyanide. Police say they do not suspect criminality at this time