Stepping up investigations into the failed UK terror plots following the detention of a doctor from Bangalore, Australian police quizzed and later released another four Indian physicians, who had earlier worked with the British health system.

The four doctors were interrogated in the western cities of Perth and Kalgoorlie after police carried out raids in hospitals there. Australian Federal Police Commissioner Mick Keelty said four search warrants were executed at hospitals in Kalgoorlie and Perth.

Attorney-General Philip Ruddock said a number of items have been taken by police for further examination, including telephones and laptop computers. No dangerous materials have been seized, he said. Keelty said the four Indian doctors were released after questioning. He said the four migrant doctors had worked in the British health system and were not considered suspects and had not been charged.

Widening it probe to a third state, the Australian police questioned the fifth migrant doctor in New South Wales. Keelty said the fifth doctor worked in Sydney. Some media reports stated that he too was an Indian.

Acting New South Wales Premier John Watkins said he had no comment at this stage about the doctor who was questioned in NSW by the federal police.

Western Australian Deputy Commissioner (Specialist Services) Murray Lampard said there was nothing to suggest a heightened risk to the public "at this point".

"The persons of interest have been cooperative and I want to emphasise that if police held information that suggested a heightened risk to the public, the public would be informed, but that is not the case on this occasion," he said.

Keelty said that there was nothing to suggest that those who have been inspected in Australia are guilty in an offence. "We are trying to establish what the linkages are and whether there is any criminal aspect to those linkages."

"There are some principals who are innocent, who couldn't be guilty. There are a number of people party in this investigation. It doesn't make them the suspects but it is quite a complex investigation in the links to make them concrete."

Keelty said the mobile phones and laptop computers, which have been seized, contained about 31,000 separate documents.

"We have now, with the West Australia police, executed warrants in Kalgoorlie at the hospital and also at Royal Perth Hospital," he said. "A total of four search warrants have been executed in West Australia."

He said the doctors who were questioned in Western Australia were overseas trained and of similar backgrounds to Indian doctor Mohamed Haneef, who was being detained in Queensland.

The police custody of Haneef, who hails from Bangalore, was yesterday extended by four more days for further questioning in the failed terror plot in London [Images] and Glasgow.

In a closed court hearing in Brisbane yesterday, the Australian Federal Police was given another 96 hours to continue holding Haneef, who has been under detention without charge in Brisbane since Monday.

Haneef is one of the eight persons -- one in Australia and seven in the UK -- being detained by police in connection with the failed terror plots.

Commissioner Keelty said none of the five doctors being questioned had been arrested.

"What we want to do is reassure people that this is not an investigation into the medical practitioners per se," he said.

"It's in support of the investigation by the London Metropolitan Police counter-terrorism command. It's important that we all realise that what we're doing here in Australia is supporting that bigger investigation in the UK."

Western Australian Premier Alan Carpenter also said that the investigation did not mean there was a heightened security threat in Perth. It was a "routine follow-up inquiry, he said.

WA Secretary of the Australian Medical Association Geoff Dobb said the four doctors questioned by police in Western Australia were not employed through the organisation.

"It is likely that they were recruited directly through the government public hospital system."

Attorney General Ruddock said that despite today's investigations there was still no information to suggest an increased threat of a terrorist attack in Australia. "There has been no alert level change."

"While the investigations are ongoing, they are not investigations that have led us to form a view that a terrorist attack in Australia is probable."