Three university students and a man have died in suspected drugs-related incidents in the north-east of England.
Two 18-year-old women and a 21-year-old man died in Newcastle, and another man, who was also 18, died in Washington, Northumbria Police said.
One of those who died has been been named as Newcastle University student Jeni Larmour, 18, from County Armagh.
Police said ketamine and MDMA were "suspected to have been a factor in the deaths of the students."
A large-scale investigation has been launched and officers have been searching student accommodation with sniffer dogs.
Ten people have been arrested and released on bail as inquiries continue.
Ms Larmour had been "a model pupil" and deputy head girl at The Royal School Armagh, the school said in a statement.
"Her outstanding qualities as a pupil were recognised in her final year when she was appointed deputy head girl, a role she carried out to a very high standard," it said.
Newcastle University said its students had been in the city for less than 48 hours when they died.
Vice-chancellor Chris Day has written to all students warning them about the two tragedies.
"We are all heartbroken and our thoughts and condolences are with their families, friends and loved ones at this most difficult of times.
"We know that many of you will be affected by this distressing news.
"Whatever difficulties you have gone through, we have ample support both at the university and in the city," he said
"Whatever those problems are, please do not turn to excessive alcohol or drugs to solve them because you have seen the potential consequences."
The university's students' union urged students, and all young people in the area, to "look out for each other".
Ms Larmour was pronounced dead at a building on Richardson Road shortly after 06:00 BST on Saturday.
Later on Saturday, at about 16:00, emergency services were called to a man on the Coach Road Estate in Washington who had suffered a cardiac arrest after reportedly taking MDMA, police said.
On Sunday, at about 08:15, police were called to Newcastle's Melbourne Street when a 21-year-old Northumbria University student became ill. It is believed he had taken MDMA. He died in hospital.
Then just after 13:00 on Sunday, another 18-year old female student was found dead at the same student building where Ms Larmour had died the previous day. Police said it was "believed ketamine had been present at the address".
Ch Insp Steve Wykes said it was "too early" to say whether a "bad batch of drugs" was involved.
He added: "What we must remember is illegal drugs are never safe and so that message is incredibly important.
"But we are conducting significant inquiries to try and understand what the substances involved do contain."
'Concern about nightclubs'
Professor Fiona Measham, chair in criminology at Liverpool University and co-founder of the harm-reduction charity The Loop, said she believed lockdown restrictions were of "concern".
"Particularly because nightclubs are closed and the pubs are closing early," she told BBC Radio Newcastle.
"I think the reason it's a concern about nightclubs in particular is that nightclubs often have paramedics, they have harm-reduction services and they have security staff that help keep people safe.
"So if you close the nightclubs, you lose that safety net."