Six months after her foster son, Maxim Vorobyev, went missing, Grace Pace still keeps his mobile phone with her at all times.

Just yesterday, the heartbroken woman pressed a button on the phone by mistake and the music of Rihanna, on whom 16-year-old Maxim had a teenage crush, reverberated from the handheld device.

"I think about him all the time. Around this time I was speaking to him on the phone," a distraught Mrs Pace said yesterday.

Maxim disappeared on February 25 - six months to the day - and his body was retrieved from the sea in Valletta 10 days later.

Crying inconsolably, Mrs Pace recounted the last time she spoke to the Russian orphan who she and her husband, Albert, had fostered for seven years. She had asked him to take some jelly to her brother's shop in Republic Street so that her brother could in turn take it to hospital for Mrs Pace's uncle, who passed away days later.

Maxim had left the house, in Valletta, at about 6.30 p.m. but he never made it to the jewellery shop and was never seen afterwards, triggering a search that ended in tragedy.

"To me he was my son. He was never a boy I fostered," she said.

Mrs Pace still wants to know how Maxim drowned in Grand Harbour: "Nobody told us what led to his death. It's still a mystery for us and it is very hard not to know exactly what happened.

"It is already hard enough that I lost him but it's even harder not knowing what led to his drowning," she said, although she had words of praise for the police who had searched high and low for the missing boy.

An autopsy had concluded the boy's death was "compatible with drowning" and forensic experts said no signs of violence had been found on the teenager. But for Mrs Pace it remains a mystery and she longs to know how her son ended up in the sea.

"It hurts not to know," she said.