The unnamed 32-year-old, who is employed in the workshop at HMP Belmarsh, was arrested on suspicion of misconduct in public office.
She is said to have grown close to Jordan McSweeney, 29, who is serving at least 38 years for the murder of 35-year-old Zara.
The woman was arrested on April 6 last year after cops received an allegation of inappropriate conduct.
She was bailed bending further enquiries and later released under investigation.
A Ministry of Justice source said: “There is zero tolerance of inappropriate relationships with prisoners.
“Those found guilty can end up in prison themselves.”
Another source said: “She was seen behaving intimately with McSweeney, who is one the most dangerous prisoners.
“Just look at his record.
“How you could go anywhere near a man like that romantically is beyond me.”
The Prison Service said: “It would be inappropriate to comment on a live police investigation.”
It is understood that the worker has been suspended from the Prison Service.
Staff found guilty of inappropriate relationships with lags can be sent to prison themselves for misconduct in public office.
McSweeney had been released from a prison sentence just days before he pounced on Zara Aleena as she walked home in Ilford, east London.
His licence was revoked after failing to meet probation officers but he was not recalled to prison.
This meant the predator, who was jailed for life for murder, was free to roam the streets hunting for a victim.
On the night of Zara’s murder, McSweeney was captured on CCTV stalking the streets of east London.
At least four other women managed to escape the killer’s clutches as followed them in the darkness.
One even ran for safety into a home on the same street were Zara was brutally murdered.
After setting his sights on the law graduate, McSweeney dragged her into a driveway where he repeatedly kicked and stamped on her.
Zara was discovered partially clothed and struggling to breath by horrified neighbours who heard her screams.
The prosecutor said she had been attacked with “savagery that is almost impossible to believe”.
A post-mortem gave her cause of death as blunt force trauma injury and neck compression.
McSweeney was described as a “damaged” person from a troubled childhood in court.
His ex-girlfriend Stella Lowe also told how how the brute was a “ticking time bomb”.