A joint investigation is under way between Wiltshire Police and the Met's counter-terror command to investigate a possible chemical substance attack.
It comes after a man and woman were found unconscious at a house in Muggleton Road on Saturday night.
The town is roughly eight miles (13km) from Salisbury, where former Russian spy Sergei Skripal and his daughter Yulia were targeted with the nerve agent novichok in March.
A number of places in Salisbury and Amesbury have been cordoned off.
Here are the investigation points.
House in Muggleton Road, Amesbury
The pair were found at a house on a new housing development on the southern edge of the town, which lies close to Stonehenge.
Resident Chloe Edwards, 17, described seeing police cars, fire engines and people in "green suits" on Saturday night.
"We saw everything… We were just eating our dinner and all these emergency vehicles turned up.
"They were putting on these green suits and we thought it was the gas as our electricity was turned off as well."
Amesbury Baptist Centre
About a mile from the house is Amesbury Baptist Centre. This is believed to be one of the last public places the man and woman visited on Saturday afternoon.
They had been attending a community fundraiser.
Church secretary Roy Collins said 200 people attended the event, including many families and children, but "nobody else has suffered any ill effects".
Queen Elizabeth Gardens
Queen Elizabeth Gardens, in Salisbury, is also being investigated. It is about 10 miles away from the house.
On Wednesday morning, a large section of the road was cordoned off and a blue forensic tent was set up.
Harcourt Medical Centre
Right by Queen Elizabeth Gardens is the medical centre, which is also under investigation.
Property at John Baker House
Police have cordoned off a property at John Baker House – a supported housing scheme in Rolleston Street, Salisbury, for homeless people.
Boots
Two police officers have been guarding a Boots store, which was closed as part of the investigation.
Patients trying to get their prescriptions filled have been turned away.
Amesbury resident Coral said she was directed to another store.
"It's concerning," she said.
Another resident spoke of her concerns, saying: "We all asked straight away 'what is it?' After Salisbury, that was a nightmare, I don't know if this is the same or not."
Shopkeepers said police entered the Boots at about 8.30am and were not in protective clothing.
Porton Down
Samples of the substance are being sent to experts at government chemical weapons research laboratory Porton Down, five miles away.
More from Wiltshire
It is the same facility that identified the substance used in the Skripal poisoning as novichok.
A Downing Street spokesman says the incident is being treated "with the utmost seriousness".
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