Ukrainian International Airlines has announced an indefinite suspension of all flights to Iran after a Boeing 737 passenger jet crashed shortly after takeoff in Tehran en route to Kiev, killing all 176 aboard, early Wednesday morning. After the crash, Iranian officials said the plane suffered technical problems. But Iranian investigators now say the plane was on fire while it was still in the air. Ukrainian investigators are looking into the possibility that an anti-aircraft missile could have hit the passenger jet or that there was an engine malfunction. Iran says it is withholding the plane’s black box and will not give it to Boeing or the Americans. The jet crash came only hours after Iran launched missiles at two Iraqi military bases that house U.S. and Iraqi troops as revenge for the U.S. assassination of Iranian commander Qassem Soleimani last week. The jet was a predecessor to Boeing’s flawed 737 MAX passenger jet, which has been grounded worldwide following two deadly crashes that killed 346 people.