Two teenagers have drowned in front of family members in the Norfolk Broads after “no swimming” signs that had only been in place for a couple of days were vandalised.

Bonheur Musungay, 14, and Stella Kambi, 17, died on Wednesday after getting into difficulty in St Andrew’s Broad on the Thorpe Marshes reserve on the outskirts of Norwich.

A statement from Norfolk police said: “The bodies of a 14-year-old boy and 17-year-old girl were recovered from the water at about 7.20pm and despite best efforts they were sadly pronounced dead at the scene a short time later.”

Brendan Joyce, chief executive of Norfolk Wildlife Trust, which manages the reserve, said there had been a spate of vandalism at the former gravel pit site, with warning signs installed on Monday almost instantly ripped down.

He said: “We are all very shocked and saddened at Norfolk Wildlife Trust to learn of the tragic loss of life of two teenagers at our Thorpe Marshes nature reserve. Our thoughts are with family and friends. We are unclear as to exactly what took place that led to such very sad circumstances. We would not want to speculate, but rather await further information from the emergency services.”

On Thursday, only one sign could be seen at the entrance to the site, almost half a mile from where the pair were thought to have entered the water. At the beach area near where they died, only one sign could be seen warning of dangerous algae.

Gordon Copp and his daughter Amy, 14, who is in the same class at Thorpe St Andrew high school as a relative of the pair who died, visited the site on Thursday. Amy said: “This is a popular swimming spot – often kids from school will come down here and come back with wet hair. I’ve been worried about safety for a while and there isn’t anything to stop people getting into the water.”

Copp, who lives nearby, let the families of the teenagers sit in his garden as emergency services launched a major search to find the pair at about 4.50pm. “They were screaming and wailing and seemed in a terrible state,” he said. “They seemed absolutely distraught and my heart goes out to them.”

Despite a rescue operation involving more than 40 people, including fire and rescue divers, paramedics, the coastguard and a police helicopter, the teenagers’ bodies were found in the water and they were pronounced dead at the scene.

Ch Insp Nathan Clark of Norfolk police said the force had received calls from the public, as a crowd gathered by the waterside at about 4.50pm on Wednesday. “We were alerted to two people struggling in the water,” he said. “There was a coordinated effort to locate them, but unfortunately the two teenagers were found dead.

“We are very much keeping an open mind about what happened and are interviewing people at the scene, including family members. I would like to offer all my sympathies to them – I can’t imagine anything more tragic than losing two teenage kids.”

According to the NWT’s website, St Andrew’s Broad is a former gravel working that is home to a variety of waterbirds. It says there is no swimming allowed because of deep water.