Should killer Victorino Chua ever have been allowed to work as a nurse in the UK?
The 49-year-old has been found guilty of murdering two of his patients at Stockport's Stepping Hill hospital and poisoning others by contaminating saline drips and ampoules with insulin.
Investigations into the Filipino father-of-two's background have cast doubt over the validity of his qualifications.
When officers from Greater Manchester Police travelled to Manila, the capital of the Philippines, they discovered inconsistencies between Chua's two nursing certificates.
BBC North West Tonight understands investigators were so worried that they wrote to the government and immediately contacted the Nursing and Midwifery Council (NMC).
Nazir Afzal, who was responsible for prosecuting Chua, said the Stepping Hill case raises the "extremely worrying" prospect that many untrained foreign workers could be working in UK hospitals.
The former chief prosecutor for the North West believes there could be many nurses using fake qualifications in hospitals across the UK.
Nazir Afzal said: "I do not know whether there were hundreds or thousands or dozens.
"What I do know is the opportunities were there for them to lie about their qualifications, to obtain them fraudulently, and to cover up their disciplinary matters.
"It must be, for all of us as patients, extremely worrying and desperately concerning.
"In all my 24 years as a prosecutor, I have never escalated concerns to another government department except in this case," he added.
The NMC's Chief Executive and Registrar Jackie Smith said significant changes had been made to the registration process since Chua came to the UK.
"We're talking about identifying individuals who can practise safely," she added.
"I don't know of a system that can identify potential individuals who might commit the most serious crimes."
But she said she was "as assured as she can be" that the register and assessment of individuals coming to work in the UK is "as sound as it can be".
On one street in Manila, North West Tonight found evidence that any document or diploma can be forged for a price.
It is known as the "Recto University".
Nursing degrees are particularly popular and as little as £20 can buy a qualification.
One forger, who wanted to remain anonymous, told BBC News: "Lots of requests to make a diploma in nursing. On average per month 35 people are asking for that. It costs 1,500 pesos (about £22 / $34)."
Chua finished his training at Manila's Galang Medical Center, which was once listed as one of the worst performing nursing schools in the Philippines and has since been shut down.
When Greater Manchester Police flew out to look at his records they found his certificates did not match - and also questioned whether he sat his final nursing exam.
Det Supt Simon Barraclough said: "Victorino Chua has provided us with certificates from his training. We've had two certificates. Those certificates don't correspond to each other.
"Even assuming one of them is right, the other is not."
While police were unable to confirm Chua's certification was forged, the detective added: "All I can say is we have absolutely no confidence that those are bona fide qualifications."
Prosecutors suspect the person who sat Chua's final nursing exam was not Chua.
Det Supt Barraclough said: "You have to present yourself with a photo of who you are to take the exam. When I look at that photograph, to me the person who presented to do that exam does not look remotely like Victorino Chua."
The photograph was not of sufficient quality to enable facial recognition tests but police said they compared photographs of Chua from that period of time with the photos from the nursing exam.
Investigators did not think the pictures matched.
Det Supt Barraclough added: "You cannot be sure the nursing accreditation Chua provided is accurate because there's basically some chap who provides them for a fee. He can write anything on them."
Chua's qualifications show he did fail one nursing exam.
Jose Ray Fulgencio, former registrar at Galang Medical Center, told the BBC he believed Chua did graduate - but suggested his certificates could have been altered.
"I don't know if there was some changes by someone else to change their official record from this office," he said.
"Maybe it [buying diplomas] was a practice before by not only our school but some [other] schools also."
He also told BBC North West Tonight fake qualifications were "rampant".
"Not only for Galang Medical Centre but some of the universities or colleges - you can go to Recto if you want," he added.
Chua had finished his nursing qualifications in June 1989.
His first nursing job was at Manila's Metropolitan Hospital in April 1991. He met his wife at the hospital in 1992.
In February 2002 the Chua family moved to the UK.
Police also discovered Chua was disciplined at Manila's Metropolitan Hospital, which BBC News understands was for theft.
But there is no mention of this disciplinary matter on the documentation he brought with him to the UK.
He used this when applying for jobs in nursing homes and hospitals in Greater Manchester and Warrington.
There has been a crackdown on Philippine's forgers and poor nursing schools. But education officials argued Chua's qualifications and training have no bearing on what he did.
Dr Patricia Licuanan, Chair of the Commission on Higher Education, said: "I feel very strongly that the school that nurse came from, whether excellent or questionable, has nothing to do with the case."
The NMC's Jackie Smith said when Chua registered with the Nursing and Midwifery Council "he met the requirements in place at the time" and provided certificates and evidence of training.
A number of significant changes have been made to the vetting process since Chua arrived in the UK in 2002, Ms Smith added.
Supervised practice for nurses was introduced in 2007 and more stringent checks of documentation were added to the process in the last two and a half years, she added.
She said: "Right now anyone coming from abroad [outside the EU] has to do an online test first of all. If they pass that test they have to obtain a visa via the Home Office.
"If they manage to get a visa then they have to come here with their original documentation. We have to have a face-to-face interview. We carry out checks on that documentation. They must take a test of competence.
"If they get through that test - a skills test - they go on our register.
"That's very different to the process in place in 2002."
She added: "There are a number of hurdles any applicant would have to overcome before getting on our register. If there are concerns about an individual there are processes in place to pick that up."
If Victorino Chua was unqualified it did not make him a killer.
But if the problems with his accreditation had been discovered and that had prevented him from working in the UK, he would never have been able to murder and poison patients.