Six young people were rushed to hospital after taking drugs at a bank holiday flat party in Manchester last weekend.
Police and paramedics were called to the property in Heaton Moor, Stockport, just before 5am on Sunday and initially three people were taken to hospital.
About an hour later — as a police investigation was beginning — more people who were at the party became ill and paramedics were called to Shaw Road a second time.
Three more casualties were taken to hospital, Manchester Evening News reports.
Of the six who had become ill, three were in a serious condition and had to be placed on ventilators. They are all believed to be recovering.
A man, 22, was arrested by police on suspicion of possession of drugs with intent to supply. He has since been released under investigation.
A woman, also 22, was also arrested and later accepted a caution for possessing cocaine.
Deputy Mayor of Greater Manchester Beverley Hughes referred to the incident at a press conference primarily about Trafford and Bolton remaining in local lockdowns on Wednesday during a round-up of police activity during the bank holiday weekend.
She revealed GMP officers had to deal with "a very significant period" across four days, from Friday to Bank Holiday Monday.
Baroness Hughes, who has responsibility for overseeing policing in Greater Manchester, continued: "And three young people [were] on ventilators over the weekend after taking what we think was LSD at a party."
LSD, also known as acid, is a class A drug, so illegal to possess or sell, and acts as a hallucinogen.
On Thursday Greater Manchester Police confirmed details of the incident, including that six people fell ill, three of them seriously.
A spokeswoman said none of the casualties were now in a life-threatening condition.
Detectives are appealing to anyone with information about the party to get in touch.
One resident described seeing a number of police cars and officers and forensics specialists focusing their attention on a first floor flat on Shaw Road during the early hours of Sunday.
Another said: "This is a nice family area but there are a few places around here where there have been parties during lockdown and it can get out of hand."
A spokeswoman for North West Ambulance Service (NWAS) confirmed two ambulances were initially called to a property on Shaw Road at 4.42am on Sunday when three casualties were taken to hospital.
NWAS received a second call at 6.03am and this time sent a rapid response vehicle and two more ambulances to the scene.
Three more casualties were taken to hospital, the spokeswoman confirmed.
A spokeswoman for GMP said: "Enquiries remain ongoing."
Anyone with any information is asked to contact police on 0161 856 69976 quoting incident number 685 of 30/08/20 or call Crimestoppers, anonymously, on 0800 555 111.