Police have called for calm in the wake of the brutal murder of a Saudi Arabian student.
Nahid Almanea died after she was stabbed 16 times while walking along a path in Colchester, Essex, on Tuesday morning.
The 31-year-old had been taking an English language course at the University of Essex since January as part of her studies for a life sciences PhD.
A 52-year-old man arrested in connection with her death has now been "positively eliminated from inquiries", Essex Police said, and was last night released.
There are fears that the killing could be linked to the murder of James Attfield, a vulnerable man with brain damage, who died after being stabbed more than 100 times in a park in Colchester in March after detectives said there were "obvious similarities".
Police are also investigating whether Ms Almanea may have been targeted because she was wearing a Muslim abaya robe and a multi-coloured hijab headscarf.
Officers said there are other possible motives but called for calm, saying they are aware of and investigating threats of revenge attacks made on social media.
Chief Inspector Richard Phillibrown, the town's area commander, said: "I see this murder as an attack on the entire community of Colchester and I believe we will all pull together to respond to it.
"I am confident that the vast majority of people in this town will pay no attention to those trying to stir up trouble."
Essex Chief Constable Stephen Kavanagh and the county's police and crime commissioner, Nick Alston, issued a joint statement, saying it is not yet known if the latest killing was motivated by Ms Almenea's religion.
They warned against jumping to conclusions, saying this would be "deeply unhelpful, wrong and dangerous".
The statement added: "Very sadly, we have now had two murders in recent months in Colchester.
"We would urge everyone to work with Essex Police, who will, supported by local communities, do everything possible to find the person or people responsible for both these awful crimes.
"I know that everyone in Colchester will come together, and remain strong and united in ensuring that the town and the district continues to be safe and welcoming to all."
Mr Phillibrown said he was working with leaders from across the community.
"What we would also ask is for people to look out for each other, for neighbours to keep an eye on the vulnerable and for everyone to remain vigilant," he said.
"From today we will be putting extra officers in to Colchester and we intend to have additional patrols across the town, especially in public open spaces.
"They are there to offer reassurance to people and to speak to them about any concerns they may have or take any information they want to pass on.
"Our advice to people is that we have now had two knife murders in Colchester in less than three months where the motive for the attacks remains unknown.
"Against that background we would advise everyone in Colchester to remain vigilant.
"Both of these attacks were on lone people who were in locations where it appears no-one else was nearby at the time.
"For that reason we would remind people to take sensible precautions, that we would always recommend, to stay safe. This includes avoiding situations in which you could find yourself isolated and alone in a public area."
Mr Phillibrown said the force was aware that threats of revenge attacks had been made on various social media sites.
"These are being looked at and, if there is evidence of criminal offences within the UK, they will be pursued," he added.
"We will take immediate action against anyone who commits any offence."
The force released CCTV footage taken from a local newsagent which showed Ms Almanea's last movements as she headed for the footpath on Tuesday morning.
They also released a map of her likely route from her home in nearby Woodrow Way, where she lived with her brother, to the Salary Brook trail, and a photograph of the bag she was carrying.
Anyone with any information that could assist the inquiry is asked to contact the Major Investigation Team on 01245 282103, Essex Police on 101 or contact Crimestoppers, anonymously, on 0800 555 111.