One of the survivors of killer nurse Ben Geen has called for an independent investigation to discover why it took so long to catch him.

On Tuesday at Oxford Crown Court, Geen, 25, from Banbury, was convicted of murdering two patients and causing grievious bodily harm to a further 15 while working in the accident and emergency ward of the Horton Hospital in Banbury.

The former territorial army soldier got his kicks by injecting patients with a variety of drugs which caused them to stop breathing, so he could revive them. The incidents all occurred between December 2003 and February 2004.

During the trial the prosecution said that unexplained respiratory arrests were very rare, but it was only after the case of Timothy Stubbs, Geen's 17th and final victim, that an investigation took place.

Robert Robinson, 55, of Oxford, who was Geen's third victim, yesterday asked for health chiefs to conduct an independent inquiry to find out how Geen was able to drug so many patients before suspicions were formally raised.

Mr Robinson said: "I think an enquiry has got to be looked at. How was this guy able to get his hands on prescribed drugs for so long without other people knowing?

"He had already made comments to staff on the lines of 'it's amazing that every time I work someone has a respiratory arrest'. Why did it then take so long for staff to investigate the matter further?"

Banbury MP Tony Baldry has backed Mr Robinson's call for an independent inquiry, and said he is writing to Oxford Radcliffe Hospitals NHS Trust chief executive Trevor Campbell-Davis.

Mr Baldry said: "The trust may say it all happened over a period of a few months, but the fact that these are such rare occurences makes me wonder whether questions could have been raised earlier.

"It is difficult to investigate these things when there is a court case happening, but now that is out of the way, I would have thought it would be sensible for a short independent inquiry to take place. There may be lessons which could be learned for the trust as a whole."

Following Geen's arrest on February 9, 2004, an internal inquiry was immediately conducted at the Horton Hospital, and a report completed by September 2004.

It included looking at the way notes were recorded by doctors and nurses and the accessibility of drugs cupboards to unqualified members of staff.

A further investigation was carried out by the Central England Audit and Consultancy board between November and December 2005 which looked at procedures and practices and made additional recommendations.

Helen Pegg, spokesman for the Oxford Radcliffe NHS Trust, said the Trust would co-operate fully with any further investigation, but did not feel it was necessary.

She said: "We feel that he Geen was picked up very, very quickly, far more quickly than some other cases and he was identified through our rotas immediately it was realised that something was happening.

"Our feedback from the police was that it was that the investigation was very thorough and there was no way it could have been picked up any quicker."