A Eurofighter Typhoon jet suffered a "system failure" whilst flying over Shropshire, the RAF has revealed.
Trails were left by the plane, said witness Andy Brooks
The plane had to drop 30,000ft (9,144m), which inadvertently caused it to exceed the speed of sound.
This caused a sonic boom, which many people in Shrewsbury thought was an explosion or an earthquake.
Sqd Ldr Martin Locke said: "The cause of the system failure will be investigated." He apologised for any anxiety caused.
The incident happened at 1606 GMT. The RAF said there were two people on board the Eurofighter, which had flown from RAF Coningsby on a training sortie.
The aircraft was successfully returned to Lincolnshire and landed at 1645 GMT. "Neither of the aircraft occupants were harmed during the incident," said the RAF.
RAF spokesman Martin Locke said the aircraft made an emergency call to Air Traffic Control and in order to ensure the safety of the aircraft and its occupants, performed an emergency descent to 10,000ft (3,048m).
In the course of this manoeuvre, the aircraft broke the sound barrier, causing a sonic boom, he said.
The "boom" was heard across Shrewsbury, causing BBC Radio Shropshire to be inundated with calls from anxious listeners.
One of the callers, Jackie Cassidy said: "It was really frightening."
She said she heard a bang, the windows shook and the door came ajar.
Mrs Cassidy, who works for Hatchers solicitors in Shrewsbury, said: "Nobody knew what was going on and everyone poured out on to the streets."
Witness Andy Brooks said: "After the boom, the clouds just parted and the [smoke trail] is what I saw."
BBC Radio Shropshire's news editor Sharon Simcock said: "We thought someone had driven into our building."
In a statement, the RAF said: "We regret any anxiety this incident may have caused to the local population."