A German woman has been killed in a shark attack while snorkelling off the Egyptian Red Sea resort of Sharm el-Sheikh, officials say.
The death comes after four people were injured in similar attacks at the resort earlier in the week.
The authorities had reopened the waters after saying they had captured the sharks involved in the earlier attacks.
Officials say they are baffled by the repeated attacks and are consulting marine biologists.
There are fears about the impact of the shark attacks on the tourism trade in the world's most popular diving spot.
Egyptian officials said the elderly German woman had died immediately after the attack, in which she was reportedly bitten on the thigh and arm.
After last week's attacks - in which three Russians and a Ukrainian were injured - the environment ministry caught and killed two white tip sharks and displayed a photo of them.
Nearby beaches were reopened after authorities deemed there was no further threat.
But divers and conservationists who compared the picture with one of the attack shark, taken shortly before one of the previous attacks, said it was not the same animal.
Now all the resort's beaches have been closed again for watersports, with the exception of Ras Mohammed, a nature preserve south of the city, Egypt's Tourism Minister Zuhair Garana told AFP news agency.
"We are getting marine biologists from abroad to assess the situation and why there was this change in biological nature," he said, referring to the repeated nature of the attacks, which some experts say is unusual.
"This is unnatural. It has never happened before," he said. "We have no explanation."
Shark experts and local observers have offered a number of possible explanations for the attacks. Some say overfishing in the Red Sea may have driven sharks closer to shore.
Meanwhile, some said predatory sharks could have been drawn to the area after a ship carrying Australian sheep and cattle for sacrifice during last month's Muslim festival of Eid al-Adha dumped the carcasses of animals which had died dying during the voyage.
Diving instructor Peers Cawley, who has lived in Sharm al-Sheikh for seven years, told the BBC that sharks were not normally seen at this time of year.
"We may see them in the summer months. This is a rare occurrence - there have been more sightings last week than in recent years," he said.
"A couple of weeks ago an Australian cargo ship dumped dead sheep that washed up around the shores. The authorities are trying to clamp down on this. Whether there's a link or not we don't know.
"The other issue that is a concern is illegal fishing. Sharm el-Sheikh has a national park, but some Bedouin may illegally fish tuna etc, to sell. So there may not be enough prey for the sharks to catch.
"The worry is that if there is not enough fish, then the sharks would look for alternative food."