Both black boxes have now been found after a passenger plane plummeted into a field in Ukraine, killing all 170 people on board. The discovery should help establish the cause of Tuesday’s disaster. Officials initially said lightning had probably hit the aircraft but investigators have warned against drawing premature conclusions. The plane was operated by one of Russia’s largest carriers, Pulkovo Airlines. Russian Transport Minister Igor Levitin, who heads a government commission investigating the crash, emphasized that Russian and Ukrainian authorities are working together.