US police have offered a $15,000 (£11,000) reward for information that will lead them to find the gunman responsible for a random bullet that struck and killed a British astrophysicist.

Dr Matthew Willson, a 31-year-old from Chertsey in Surrey, travelled last month to visit his girlfriend on the outskirts of Atlanta, Georgia.

In the early hours of Sunday 16 January, just days after he arrived in the US, a bullet pierced the wall of her apartment, travelled through the headboard of a bed and struck Dr Willson in the back of the head.

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‘I saw that he was slumped over’

Couple woken to more than 30 gunshots

Officers had been called to a nearby apartment building shortly before Dr Willson was shot after multiple 911 calls reported gunfire there, police said.

The couple were woken up in the middle of the night by the sound of more than 30 gunshots coming from a nearby apartment complex, Dr Willson’s girlfriend Katherine Shepard said. Moments later, Dr Willson was shot.

“I held him for another 20 minutes while we waited for the ambulance,” she told network WSB-TV.

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“And while we were waiting, there were more gunshots fired.”

‘Reckless discharge of firearms’

Brookhaven police said the death was a random act resulting from “the reckless discharge of firearms”.

“He was supposed to be here for three months because we’ve been long distance for a while,” Ms Shepard said.

“I picked him up from the airport, took him to his favourite eating location, and the next day, he’s gone.”

The couple had been dating for three years.

‘Much-loved’ astrophysicist

Ms Shepard’s flatmate, Aman Kar, said he has heard “a few shots” in the area in the past, but this time there were “multiple rounds”.

“It is unusual, Brookhaven is considered one of the safest neighbourhoods in Atlanta so that’s why we, Katherine had moved there to begin with, so hearing this was not normal,” Mr Kar told Sky News.

Dr Willson was a former PhD student at the University of Exeter and was a “much-loved member of our astrophysics team”, a university spokesman said in a statement.

The Foreign Office has confirmed it is supporting Dr Willson’s family and is in contact with authorities in the US.