A second man has been arrested in connection with the murder of nursing assistant Cheryl Moss.
Cheryl Moss was found stabbed in a nearby park
The 33-year-old was repeatedly stabbed while on a cigarette break behind St George's Hospital in Hornchurch, east London, on Thursday.
An 18-year-old was taken to a north London police station for questioning after a blue Nike bag was found near a river in Upper Rainham Road.
A man arrested on Thursday night has been released without charge.
Police said a post-mortem examination had confirmed Mrs Moss had died from repeated stab wounds.
Det Chief Insp Dick Langley said: "We are appealing for witnesses who may have seen a man carrying a blue Nike bag in the area of Upper Rainham Road yesterday afternoon."
The bag, which was found by a member of the public, is now being tested by forensic experts.
Mrs Moss had worked on the hospital's geriatric ward for more than a decade and was described as a popular and respected member of staff.
She had gone out for a cigarette break in a park behind the hospital on Thursday morning when she was repeatedly stabbed.
She was found at about 1100 BST and an ambulance was called but she was pronounced dead at the scene.
No murder weapon has been found and police said there was no evidence of a sexual attack.
Security has been stepped up at the hospital to reassure staff, who are said to be deeply shocked.
Trainee nurse Imelda Distura, 34, said: "Everybody is upset... I am frightened really. We are asking for more security."
Mrs Moss's family visited the hospital on Friday to lay flowers and read tributes.
Police have interviewed staff at the hospital
Their message, pinned to a bouquet, read: "Cheryl. Taken so cruelly from us. Will always be in our hearts. Dad and June."
Ralph McCormack, head of Havering Primary Care Trust, paid tribute to Mrs Moss, and said his thoughts were with her family, who have been too distressed to speak publicly.
"Cheryl dedicated herself to the support and care of others. She's been working in health services support for the last 17 years.
"She was very well respected and liked and known by her colleagues and had a great deal of admiration and respect from them.
"Clearly her loss will be a grave one."
The hospital is running most services as usual, although some staff who worked with Mrs Moss have not gone into work.