The swimming cap of the 9-year-old girl who drowned along with her dad and brother at a Costa del Sol resort has been found inside the pool’s pump system – suggesting that the three family members were sucked to their deaths, according to a report.

The Christmas Eve tragedy at the Club La Costa World in Las Lagunas de Mijas near Málaga unfolded when the British girl got into difficulties while swimming in the pool with her 12-year-old sister.

Her 52-year-old father and 16-year-old brother tried to save her but also drowned after going under, officials have said.

Autopsies performed at Malaga’s Institute of Forensic Medicine have confirmed that the three drowned – but the exact circumstances of their deaths remain a mystery, according to the UK’s Sun.

During the post-mortem, investigators found no signs of external injuries, or evidence they had been poisoned, the news outlet reported.

The discovery of the swimming cap by police divers comes amid speculation that suction problems turned the pool into a death trap, according to local newspaper Diario Sur.

A pool expert quoted by the paper described as “very remote,” but not impossible, a situation in which drains at the bottom of a pool floor could suck people underwater if skimmers and other cleaners malfunctioned.

Local media reports say a resort worker who went into the water to recover the bodies said he also had difficulties returning to the surface.

“We are carrying out multiple lines of inquiry, including looking at the swimming pool pumps,” a source at the Civil Guard told The Times of London.

“Police divers were there after the accident on Tuesday checking over the pool’s operating systems,” the source added. “At this point everything indicates the deaths were accidental, we believe by drowning. What happened was a truly terrible thing.”

Resort officials have said they were told by authorities that there was no malfunction in the pool’s circulation system, insisting that a police probe has found “no concerns” with the pool, The Sun reported.

It also has been claimed that the three died as a result of a temporary suction problem caused by a freak mishap that the resort’s management could not have foreseen, according to the news outlet.

Resort operator CLC World Resorts and Hotels said in a statement: “All at Club La Costa World resort are devastated by the tragedy that unfolded on Christmas Eve where a father and his two children were found unresponsive in a swimming pool and despite the best efforts of our first response team and the emergency services, could not be revived.

“The Guardia Civil have carried out a full investigation which found no concerns relating to the pool in question or procedures in place, which leaves us to believe this was a tragic accident which has left everyone surrounding the incident in shock,” it said.

“Naturally our primary concern remains the care and support of the remaining family members. We would therefore request that their privacy be respected at this traumatic time.”

Resort officials said the Civil Guard also has authorized the pool’s reopening – in what they say is an indication that police do not believe it poses any risks, The Sun reported.

Meanwhile, counselors are now comforting the widowed mom and her daughter — and relatives have flown from the UK to be by their side.

There have been several cases of people drowning as a result of accidents involving pumping systems in pools.

In August, Alisa Adamova, 12, of Russian, died 11 days after she was trapped underwater when her arm was sucked into a pool pump at a Turkish resort.

In 2009, British student Nathan Clark drowned at a Thai water park after apparently being sucked into a pool’s pumping system.