Last Thursday it seemed the miscreants had been caught and disaster averted. After a spate of highly unusual and shocking shark attacks in the Egyptian resort of Sharm el-Sheikh, the authorities had been on the hunt for the culprits, and it appeared they had triumphed quickly and cleanly. Four attacks had taken place on Tuesday and Wednesday, with three Russians and one Ukrainian left with terrible injuries; one reportedly lost her right hand and left leg in the incident, and another sustained injuries to her legs and back, also lost a hand, had a heart attack and had to be resuscitated.

The attacks were attributed to two sharks, which were soon brought in by a mission mounted by the environment ministry, involving a large group of conservation specialists. Footage showed one shark, an oceanic whitetip, being hauled on to a small fishing boat – its dead body cut and bloodied – hoisted by its eye sockets, turned over, its belly slashed open. A very public, very decisive slaying. It was said that this 2.25 metre-long adult shark matched the description of one seen by an Egyptian diver who had rescued one of the injured tourists; it apparently had the same damage to its dorsal fin. Case closed. All that was left to do was to send the dead sharks to be examined for research purposes and check out the contents of their stomachs, before they were embalmed and displayed to visitors at Ras Mohammed national park. The winter-sun season – that tranquil round of reading, sunbathing and snorkelling for which millions of tourists a year flock to the Sinai shore – could continue.

The next day, Nick Treadwell set off to Ras Mohammed for a diving trip. He had been told that most of the main diving sites were still temporarily closed, "but the national park was open, where the corals are amazing, and the fish are incredible". Treadwell works in finance in London, and had been in Sharm el-Sheikh for a few days for a short break with his girlfriend. An experienced diver, he set off on a boat trip with 10 snorkellers and a couple of other people who, like him, would be scuba diving. "We did our first dive of the morning, and that was great," he says. "I swam through some caves, saw some beautiful fish and corals, and then came back to the boat."

The second dive of the day was much less placid. Treadwell was down about 14 metres when his instructor noticed a stray piece of equipment below them and dived for it, before flinching in shock on his way back up. Above the scuba divers, and below the snorkellers, a large shark, more than two metres long, was slowly circling the group. When it was brought to Treadwell's attention, he says he went calm – very calm – as he watched the instructor free dive down a few metres and then start corkscrewing to the surface, blowing bubbles, to try to scare the shark away. "He went to the top and shouted, 'Shark, shark, shark – everyone get to the reef.' Everyone started swimming as fast as they could, because the reef was too shallow for the shark, so it would be a safe place. But there was an older lady, probably in her late 60s, who was slightly hard of hearing, and she was delayed. The shark started coming towards her, and she ended up kicking it in the face a couple of times, and using her underwater camera to whack it over the head. She got away, but she had cuts all over her legs. I don't know whether the shark had actually bitten her, but they looked like lacerations – almost like injuries from where she'd kicked the shark in the teeth."

Two days later, another tourist wasn't so lucky. An elderly German woman, snorkelling just a short way off the Sharm el-Sheikh coast, was bitten on the thigh and arm and died almost immediately.

Ellen Barnes, a British tourist who was in the water at the time, described the scene vividly. "The water was churning like I was in a washing machine," she said. "I was being thrown around in the blood. The shark was thrashing and tearing at this poor woman and I could barely keep my head above the water."

The shark threat that had apparently been quashed was suddenly alive again and more terrifying than ever. Speaking yesterday, the South Sinai governor, Mohammed Shosha, seemed to admit as much when he said: "We did catch the sharks," before adding, "There is another shark."

The attacks are a strange, sudden and frightening occurrence for a resort known for the beauty of its waters, and its safety. The last shark fatality in Egypt, the first in five years, took place in January, but was just south of Marsa Alam, a remote diving area a long way from Sharm el-Sheikh, where such attacks are almost unheard of. Simon Rogerson, editor of Dive magazine and author of the book Dive Red Sea, says that if someone had asked him a few weeks ago about the resort: "I'd have said it was very safe. Generally, it's a really good place – it gets very crowded because it's relatively cheap for Europeans, but the coral environment is very healthy, very beautiful, the water's clear, and it's sunny. Whatever is going on there is an anomaly, and we'll only find out in time what's really happening." The Egyptian authorities will be hoping for a verdict very soon; it is thought that beach tourism contributes 66% of the $12.3bn (£7.8bn) that Egypt's travel industry will bring in this year.

Besides the location, another factor that has sparked surprise has been the nature of the culprit. As the shark hunt showed, it's thought that the species behind these attacks is the oceanic whitetip, which isn't usually associated with such incidents – the 60 or so shark attacks that take place around the world each year are much more likely to associated with great whites, tiger or bull sharks. Ali Hood, director of conservation for the Shark Trust, says that the number of attacks attributed to oceanic whitetips since records began being compiled way back in 1580 is "fewer than 10. So we're talking about extremely low numbers."

Oceanic whitetips aren't known for active aggression then, but they do have a reputation – rightly or wrongly – for preying on humans when they are already vulnerable. In cases of shipwreck, or planes crashing into the sea, it has entered the popular imagination that these sharks often end up circling and killing people. Rogerson photographs sharks, and he says that while almost all he has encountered have been "really, really timid," oceanic whitetips do seem to be slightly bolder than their fellow Red Sea sharks, such as makos, "which like to stay cryptic. The oceanic whitetip inhabits the desert of the open ocean, they're usually found around off-shore sites, and some scientists have posited that because of their nomadic lifestyle they've become bolder than other sharks. Most sharks are very, very shy of anything that's not immediately familiar to them as a prey item, but oceanic whitetips perhaps have to investigate potential food items with more urgency than a shark that has a territory along a specific stretch of reef."

"I've heard reports of these approaches getting more and more aggressive," he continues. "I think the thing is that the longer you spend in the water with this particular shark, the bolder it gets, and the more it tries to test you. To most sharks, human beings aren't edible, and I've no idea whether an oceanic whitetip could thrive off a human. But these sharks have got a history of being a danger to people who are in the water for a long, long time." He suspects that scuba divers might not be at so much risk as snorkellers, "because they have more control, and more awareness, of what's around them. Also, in nature, if something's floating on the surface, it looks as if it's injured, so it becomes more of a target. These attacks have been happening to snorkellers who are just pootling off resort beaches, and the chances are they're not wearing wetsuits, there's all that white flesh and that seems to attract sharks – it's the colour of fish flesh, after all. Most sharks like to sneak up on their prey, they don't like being seen, and that's easier if someone's snorkelling."

That helps explain the species under suspicion, and its tactics, but it doesn't answer why these attacks are happening right now. Hood says that "to have a cluster of events, a spate of attacks of this nature in one place, is extremely surprising. What is important to consider is that these attacks were probably triggered by a particular activity or event, something man has done, rather than something that the sharks have initiated. Something new has occurred for them to behave in this manner."

Over the last week, one theory after another has been suggested. One put forward by the director of South Sinai Conservation, Mohammed Salem, is that the attacks stem from uncontrolled fishing in popular diving areas – that sharks are having to become bolder and more aggressive in their hunt for prey, owing to a growing shortage of their natural food in the ocean.

Another is that they're being attracted to humans as a result of baited dives and "chumming". This is the practice whereby fish blood or flesh is placed in the water to attract sharks for people who are involved in organised dives. Rogerson says that baiting is "illegal in the Red Sea because of the types of sharks that they have there" and that in his experience it doesn't happen. "If you were to suggest deliberately putting fishy blood and bait in the water in the way you'd see, for instance, in South Africa, you'd be politely laughed off the boat," he says. But that doesn't mean that food doesn't end up in the water. There are a great number of diving boats – often called "liveaboards" – now floating around the Red Sea, "and sharks tend to be attracted to these," he continues. "I think it's because sometimes the odd bit of food gets thrown overboard, and sharks get used to it. There's also all sorts of cargo shipping going up and down the Suez canal, and through the Red Sea into the Indian Ocean, and I'm sure they're not too bothered about little local rules about whether you can throw stuff off your boat."

There have been allegations that large cargoes of dead sheep have been dumped in the sea, after dying on route to Egypt – and it seems likely that plenty of other theories will keep swirling in the coming weeks. For now, many of Sharm el-Sheikh's beaches are back open for experienced divers, and the area is swarming with shark experts, brought over to avert further tragedy, and help secure the tourist industry. The messages coming out of the resort, from the travel industry, and government offices all over the world, are anxious and often contradictory – some British holiday companies have cancelled boat trips, while the Foreign Office has simply warned people to be on their guard. The sad truth, though, is this: only time will tell whether it's really, truly safe to go back into the water.