A woman has been arrested by detectives investigating the torture and murder of 16-year-old Mary-Ann Leneghan.
The teenager is thought to have been abducted before being killed
She was detained in the West Midlands on suspicion of assisting an offender.
Police are still questioning two men arrested on Wednesday in London and the West Midlands and another man arrested on Tuesday in London.
The arrests were a "significant development" in the hunt for a gang of up to six involved in the attack in Reading, Berkshire, on Saturday.
Det Supt Mark Warwick, leading the inquiry, said: "In the early hours of this morning we conducted further house searches in London and we are continuing to search a property in Birmingham where we also arrested a woman on suspicion of assisting an offender.
"There was a significant development in our investigation yesterday when we arrested two men, one in London and one in Birmingham."
The family released a photograph of a young Mary-Ann (right)
Three men, aged 18, 19 and 21, were now in custody being interviewed about events in Prospect Park at the weekend.
An address in London, related to one of Wednesday's arrests, was also still being searched, he said.
"A significant amount of forensic evidence has been obtained," he continued.
"We have seized hundreds of hours of CCTV footage and we have received extremely valuable information from members of the local community."
Mary-Ann's body was found in Prospect Park, Reading, on Saturday after her 18-year-old friend managed to raise the alarm.
The two teenagers had been dumped in the park after they were subjected to a prolonged violent and sexual attack by a gang of men.
The men forced them into a car after finding them chatting in the car park of the Wallingford Arms, a disused pub near their homes.
They were then driven to the Abbey House Hotel, in Connaught Road, where they were sexually assaulted for several hours.
Mary-Ann was stabbed in the neck, while her friend was left seriously injured with a gunshot wound to the head.
Their attackers then left both girls for dead in the park.
However, the 18-year-old regained consciousness and staggered to a nearby road where she flagged down a passing motorist for help.
Forensic tests were carried out in a flat above a massage parlour
She is in a serious but stable condition in hospital, where she is helping police with the investigation.
Earlier on Wednesday, police confirmed that another girl was with Mary-Ann and her 18-year-old friend earlier on the evening they were abducted.
A spokeswoman for Thames Valley Police said the girl was not attacked and has been interviewed as a witness.
Officers searched a flat above a massage parlour in the town's Oxford Road, which it is believed could be linked to Mary-Ann's death.
The latest arrest brings the total number during the investigation so far to eight.
Four men have been freed on police bail after being questioned by detectives earlier this week.