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Sharks of different species are behind a series of attacks on tourists at Egypt's Red Sea resort of Sharm el-Sheikh, a US investigator says.

Marine biologist George Burgess said a single shark had carried out two of the attacks, while a different species was responsible for two others.

Mr Burgess told the ´óÏó´«Ã½ that environmental effects had caused the "highly unusual" spate of attacks.

A German woman was killed and four people injured in last week's attacks.

Many of the main beaches have been closed to swimmers and snorkellers since Sunday's fatal attack.

The Egyptian authorities are concerned about the impact on tourism to the country, which provides a crucial source of foreign currency and jobs in the country.

Mr Burgess said the attacks were "undoubtedly" caused by environmental factors, as two separate shark species had been identified from photographic evidence provided to international investigators called in by the Egyptian authorities.

A shark hunt was useless, he told ´óÏó´«Ã½ Radio 4's Today programme, as the team had ruled out the existence of a so-called rogue shark that was acting like "a deranged human being taking lives".

"What you have here are rational attempts by a predator to find food," said Mr Burgess, of the International Shark Attack File based in Florida.

He said the dumping of animal carcasses in the area by a cargo ship last month might have contributed to the attacks by attracting the sharks nearer to shore, but said the investigation was ongoing.

In the meantime, he advised people to swim in groups in areas inside the reef, and to stay out of the water at night.

An elderly German woman was fatally mauled just metres from the shore on Sunday, just a day after beaches were reopened following shark attacks that injured two Russian snorkellers on 30 November and another one on 1 December.

The governor of South Sinai, Muhammad Shousha, said on Wednesday that one of the sharks involved in the attacks - a mako - had been caught.

But an oceanic white tip, believed to have killed the German woman, was still at large, he added.

The Egyptian government is keen to protect the tourism industry, which generated revenues of $11.6bn (£7.3bn) in 2009.