@Walter Hinteler,
The images have not yet been verified, but since there are videos and photos from different perspectives, a fake is as good as impossible.
The cause is still unclear. According to a report by the Russian state news agency RIA, a fuel tank on the bridge caught fire. In view of the pictures of the explosion, however, this can be ruled out as the cause. The burning tanks are probably a result of the explosion.
Both Russian and Ukrainian sources say that the explosion was not an accident. Recently, there have been repeated incidents of drones exploding in the Kerch region, which directly borders the bridge in Crimea.
There is a lot of speculation at the moment. It is possible that a truck exploded just as it was crossing the bridge. That's what the Russian National Anti-Terror Committee claims. However, getting a truck with an explosive device onto the bridge is not easy. Trucks are closely inspected and screened before passing the bridge.
There are also pictures showing a floating craft appearing under the bridge at the moment of the explosion, perhaps a small boat, perhaps a submarine. These images have not yet been verified, but the nature of the explosion could support this theory.
An attack from the air, for example with a missile, cannot be ruled out at present. However, such an attack seems unlikely. The type of explosion also points to this. Moreover, the bridge is strongly secured against missile attacks, among other things with stationary air defence systems. However, such systems have already failed in other Ukrainian attacks on Crimea, as well as in the destruction of the Russian missile cruiser Moskva in the Black Sea in April.