A sonic boom from a low-flying jet could be to blame for a "loud explosion" which stunned people in a Powys town.
The Ministry of Defence will be told about the bang
Windows and homes shook in Llandrindod Wells when the noise rang out at about 1100 BST on Wednesday.
Worried residents thought they were in the middle of a mini earthquake and called the police and the RAF.
But experts said the noise was unlikely to have been an earth tremor and more likely have been a sonic boom instead.
Powys Council's headquarters was one of the buildings where the bang was heard and workers reported that windows shook.
Council spokesman John Evans said: "We don't have a clue where the noise came from - it's a total mystery.
"It sounded like a lorry rumbling past the window, but our offices are not close to the main road.
"It couldn't have been thunder either as it's a clear day.
"Some people at County Hall reported that windows shook when the noise was heard."
A spokesman for the Metropole Hotel also reported hearing a "loud explosion."
"It seemed to come from the area of the park across the road from the hotel," he said.
Resident Marie Davies said she had been scared by the noise.
"It sounded like a loud explosion, although I've never heard anything like it before.
"There were some planes flying over at the time of the bang.
"Everybody in the town heard it."
Pc Clive Rees of Dyfed-Powys Police said the police station's windows shook.
"We have received several reports from people in Llandrindod and others from people living a six-mile radius from the town.
"I initially thought it was thunder, but on reflection there's no way it was thunder.
"I've reported the noise to the RAF as it could have been a sonic boom from a jet."
An RAF spokesperson said it had received one inquiry about the noise and would report it to the Ministry of Defence.
However, Glenn Ford, a seismologist with the British Geological Survey in Edinburgh, said they had had no signals "consistent with an earthquake in the area".
"There are a number of possible explanations and it could have been a sonic boom and maybe even a meteor that people heard.
"But it's more likely to have been a sonic boom."
Like earth tremors, sonic booms can cause loud bangs and can shake windows and homes, he said.