Valentina Novozhenova, 23, disappeared from a Koh Tao hotel on February 16, the day she was supposed to check out after a week-long stay.

“The initial investigation suggested no clues of an attack or murder,” said Surat Thani provincial police chief Apichart Boonsriroj. “Her latest communications via social media indicated she wanted to free dive deeper than 22.3 metres without an assistant or equipment.”

Police are seeking cooperation from diving groups in the area to help to search for her, he said.

Russian Sergey Busagin, a relative of Novozhenova, has contacted police.

Novozhenova is 1.7 metres tall and a passionate swimmer who often brought a purple towel or coat with her while diving or snorkelling, police said.

In her room at the hotel, her belongings, including a passport, clothes, bags and cellphone, were found. That suggested she intended to return to the room, police said, adding that there was no evidence of a struggle having taken place in the room.

Her communications on VK, a social network platform popular among Russian users, showed that she left Moscow for Bangkok on February 7 and arrived the same day. She posted pictures of her touring the Thai capital a day later as well as pictures of a hotel and beach in Koh Tao on February 10.

On February 11 at 8.44am, she posted a message saying her mobile phone was broken and she would have it fixed, and a minute later she posted that she was on a beach and would find something to eat.

At 10.26am, she posted that her mobile phone was working and she was heading to Chalok Beach to take pictures, with many pictures posted up to 2.45pm.

Her last posts on social media were on February 13, when she posted about cats and the Indian food roti as well as diving activities. She said she loved free diving, which involves deep dives without a scuba air cylinder.

She said that she could free dive to a depth of 22.3 metres, and posted a picture to show how deep that was.

She added that she would dive deeper next time.

Police said Novozhenova mentioned on Whatsapp that she had consulted a psychiatrist on February 3 because she feared something but could not indicate the cause. The psychiatrist advised her to enrol in long-term therapy.

In a notorious case that made international news, two British backpackers were murdered on Koh Tao in September 2014.