Church leaders say the widow of a Christian pastor from London who drowned in a pool at a Spanish resort with two of their children on Christmas Eve has astonished them with her grace and dignity in grief, as her lawyer stressed the case was not closed.

Olubunmi Diya, who witnessed the incident with their surviving daughter, Favour, 14, has returned to the UK and is being comforted by members of the Redeemed Christian Church of God (RCCG), of which her husband, Gabriel Diya, 52, was a pastor in Charlton, south-east London.

The incident has raised questions after Gabriel Diya and the couple’s son, Praise-Emmanuel, 16, drowned trying to save their daughter Comfort, nine, who got into difficulties at the Club La Costa World resort near Fuengirola on the Costa del Sol.

The family lawyer, Javier Toro, is pressing for a judge to order a full investigation after a police report published on 27 December concluded the incident was “a tragic accident, in principle caused by the victims’ lack of expertise when swimming”.

Diya’s widow has disputed this, saying in a statement that all three could swim. She said she believed something could have been wrong with the concave pool and dragged the three into the middle where it was deeper, making swimming difficult for them.

Agu Irukwu, a pastor and chairman of the RCCG board of trustees in the UK, who has spent time with Olubunmi Diya since she and her daughter returned to the UK, said: “Yes, she is grieving, but my wife and I were really amazed at the grace and dignity she is exhibiting at a time like this. We left there thinking we had been in the presence of a remarkable woman.

“The strength she is showing, the inner strength, I have never seen anything like it. I have been doing this for 28 or 29 years, and I have never seen anyone handle grief the way she has handled it.”

The couple were sunbathing at the poolside when the incident occurred. There was no lifeguard on duty. Toro has said it is “very, very rare” for three people to die in the same pool, and improbable for a tall man to drown in no more than two metres of water.

More than a week after the incident, the only tangible outward signs of anything amiss at the resort were the constant security patrols around the ochre and salmon pink apartments. A private security guard was stationed beside the pool where the three had died.

“It’s not true that the family said they couldn’t swim,” Toro told the Guardian. “The daughter was taking swimming classes at school, as is compulsory, and both ourselves and the police are waiting for a report from the school.

“The police have concluded that the accident happened because they couldn’t swim, but if I can show that they could then clearly that conclusion is false. In that case, it’s up to the judge to decide what to do next. All the surviving members of the family want is to find out what really happened.”

Irukwu said Olubunmi Diya had reiterated to him that she had not told police her husband and children could not swim. Of the questions that had been raised, he added: “Yes, there are concerns. Not just her concerns. They are concerns for anyone who has a bit of knowledge of the things that happened. We have got a good lawyer. He is doing what lawyers do, supervising an investigation and asking more and more questions.

“Those questions are necessary, just so we are clear to exactly what happened. There are concerns there might be some culpability on the part of the hotel. We will wait until the reports come out, and then we will determine what the next stage is.”

Club La Costa World said in a written statement that it was aware of Olubunmi Diya’s statement. “The claims made in that statement are directly at odds with the findings of the police report. This makes it clear that their exhaustive investigations have confirmed the pool was working normally and there was no malfunction of any kind,” it said.

“CLC can confirm that the pool remains closed out of respect to the victims of this tragedy and that guests have been, and are being, directed to other pools. Our sympathies remain with the family at what we understand must be a stressful and desperately upsetting time for them.”

A JustGiving fundraiser for the family, started by a family friend, had raised more than £13,000 on Friday.

Gabriel Diya had been pastor of Open Heavens, a small but growing congregation within the RCCG, for about 13 years. He was ordained in the UK in 2006, having first joined the church in Nigeria, said Irukwu. “He was very likeable, a great personality, very passionate about his calling. And a great, great family man. Loved his children. And a wonderful husband, a very good marriage.” His wife, known as Bunmi, assisted him in pastoring the church.

The tragedy had shocked the church, said Irukwu, and messages of condolence have arrived from around the world. The RCCG had a branch in Málaga, and the pastor there and members of the congregation arrived at the holiday complex after the incident to offer support and help.

“We just want to be there for her, support her, stand with her, and just take it a day at a time as we go through the next weeks waiting for the investigations to be completed, and then with her as they literally pick up the pieces and continue with life,” Irukwu said.