Iran Says Ukrainian Plane Was On Fire Before It Crashed

Iran and Ukraine are jointly investigating the Ukrainian International Airlines plane crash in Tehran, where all 176 on board perished. The Boeing 737-800 NG disappeared from radar shortly after takeoff. A preliminary report suggests the plane was turning back to the airport and was engulfed in flames. Investigators are exploring various causes, including engine malfunction, collision, missile strike, or terrorism. The black boxes have been retrieved, but it's uncertain if Iran or Ukraine can analyze them. Iran refuses to hand them over to Boeing or the US NTSB. Ukraine declared a national day of mourning, with memorials held in Kyiv.

What has been said about the retrieval of the plane's black boxes?

What action did Ukraine's president declare on the day of the crash?