Sousse, Tunisia (CNN) — The last time family members heard from John Stocker and his wife, Janet, the couple raved about their beach holiday in Tunisia.

“They were having a lovely time,” said Stocker’s son, Mark, “The weather was great, just relaxing.”

That was the day before Friday’s terrorist attack in which a gunman stormed the resort where they were staying, the Hotel Riu Imperial Marhaba, killing at least 38 people and injuring dozens more.

Three days later, Mark Stocker and the rest of the family back in Morden, southeast England, are still desperately waiting for news about their relatives.

“It’s like they disappeared,” he told CNN over the weekend.

In an effort to track down his father and stepmother, he has phoned the hotel to ask employees to check on them. Their luggage, passports and money were still in their room, but the couple weren’t there.

Mark Stocker has called every hospital in the area of Sousse, the town where the attack took place, but without finding any trace of his relatives.

“Where can they be? It does lead you to one conclusion, but it’s not the conclusion that you want to hear or believe,” he said.

The family has launched a social media campaign, using the hashtag #findjohnandjanetstocker, to try to garner information on what happened to them.

Fifteen of those killed in the attack were British and that number is expected to rise, UK authorities have said.

“There are a significant number of victims who have not been positively identified at this time and it is highly likely that a significant proportion of them will be British,” British Foreign Secretary Philip Hammond said Sunday, according to the Press Association.

Tunisian Prime Minister Habib Essid said the majority of the dead were British, followed by German and French citizens.

ISIS has claimed responsibility for the attack, but it is unclear if the Islamist group, whose headquarters are in Syria, had any direct role in it.

People injured in the shooting have described the terrifying scene as the gunman stalked through the resort, taking aim at bewildered tourists with his assault rifle.

Hammond said Sunday that the delay in identifying some of the victims was because they were “dressed for the beach, not carrying ID physically on them.”

In a bid to reassure visitors, Tunisia has stepped security following the hotel massacre, which followed a deadly gun attack on a museum in the capital three months ago.

Authorities will deploy around 1,000 armed security officers inside and outside tourist zones, the Ministry of Tourism and Handicrafts said Sunday, according to the country’s state-run news agency, TAP.

Thousands of tourists have been fleeing Sousse since the attack, but some vacationers are staying put at the hotel at the heart of the killing spree.

Nathan, a visitor from Norwich, England, told CNN that Tunisian jobs depend on tourists sticking around.

“It’s a very different atmosphere than what it was, very chilling,” he said. “But what we are going to do is stay here until our holiday finishes then leave.”

On Sunday, a spokeswoman for Riu hotels said around 40 tourists remained at the Imperial Marhaba and that the company intended to keep the hotel open.

Some guests lay in the sun in areas still riddled with bullet marks.

Many of those left were on a tour when the attack happened. But Folker Schumacher, a tax inspector from Stuttgart, Germany, was in the sea. He said he and his wife ran as gunfire kicked up the sand nearby.

ISIS has posted a photo of the attacker, whom Tunisian authorities have identified as 24-year-old Saif Al-Deen Al Rezgui from the town of Gaafour, about 160 kilometers (100 miles) northeast of Sousse.

The Tunisian Interior Ministry said over the weekend that the gunman specialized in electronics in pursuing his master’s degree and didn’t have any known relationship with a terrorist group.

Tunisian citizens were wounded in the attack but none were killed, authorities said, according to TAP.

On Saturday night, a large crowd of Tunisians turned out at the hotel in a display of unity after the attacks. They chanted and sang, waved the red-and-white Tunisian flag and lit candles at the spot where the dozens of victims had been shot the day before.

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