The death of a nine-year-old boy starved of oxygen during routine surgery is becoming increasingly suspicious, police said yesterday. Tony Clowes died last month at Broomfield Hospital in Chelmsford, Essex, while undergoing an operation on a cut finger.

A breathing tube used during the surgery was found to be blocked after Tony, from Dagenham, died of brain damage. Two other similar cases where the patients survived are already being investigated. Police announced yesterday that they were also studying a fourth incident with similarites and said the blockages were "unlikely to be accidents".

A 42-year-old policeman from Grays, also in Essex, was starved of oxygen while having a kidney removed at Basildon Hospital in Essex.

In another case, on 26 April, apparatus used on a 57-year-old woman at Watford Hospital, Hertfordshire, was also found to be blocked.

Police said the fourth incident occurred during preparation for routine surgery at a hospital in Dorset. Officers are now examining records of agency medical staff and others to see if there is a common link between the hospitals.

Assistant chief constable John Broughton said: "Within the tubes are foreign objects. How they got there is a matter of this investigation."

He said: "It is early in the investigation. There are lots of things we need to do. One of those will be looking at all the staff, including agency staff."

But he added that a lot of work still needed to be done and he was not in a position to say whether any charges would eventually be brought. The issue surrounding the four cases was "a very complex" one, Mr Broughton said, which would take police "some time" to resolve. His immediate concern was to put in place procedures and systems to prevent a fifth or even sixth case arising. But while he was becoming less convinced that Tony Clowes' death, and three other similar cases, were accidents, Mr Broughton said he was some way off launching a murder inquiry. In order to do so, he said, detectives had to be sure there was a "justifiable case" and that that was not the situation at this time.

"As we investigate we will draw a picture whether (the foreign objects) were there by accident or were deliberately inserted," he said. But just because he was less convinced the four cases were accidents did not mean he was necessarily hunting a murder suspect.

Meanwhile, "extensive research and work" was being carried out to find out who was in the hospitals, where in hospital they were and why they were there at the time of the four incidents. Police said they were working hard to identify potential suspects, but denied reports they had opened a murder investigation.

The latest case to emerge occurred in November last year at the Royal Bournemouth Hospital when a tube providing a 65-year-old man with oxygen became blocked while he was being prepared for routine surgery. The man was unharmed and the incident was deemed an accident by an internal investigation.

Mr Broughton said the inquiry team had now looked at the incident in more detail. As it was similar to the cases in Basildon, Chelmsford and Watford it had been linked.

All hospitals have been reminded by the Medical Devices Agency to check breathing tubing before it is used for each patient.