Police have said the body of teenager of Alice Gross was dumped in a river and weighed down by six parts of a tree trunk, as they formally attributed her murder to builder Arnis Zalkalns.

Zalkalns evaded justice by taking his own life, and his body was found by police days after that of Gross, 14, was recovered following a weeks-long search.

Scotland Yard said there was enough evidence to have charged Zalkalns with the abduction and murder of Alice, and that the probable motive was “sexual”.

Alice was last seen on 28 August 2014 after going for a walk in west London. Police believe Zalkalns, who was convicted of murdering his wife in Latvia, encountered Alice as she walked along a canal towpath.

One week later Zalkalns, 41, was reported missing from his west London home. Police took the unusual step of naming him as their prime suspect as hopes of finding the teenager dwindled.

After a month of searches, Alice’s body was recovered from the river Brent on 30 September, after it had been deliberately weighted down.

On Tuesday the head of the inquiry, DCI Andy Chalmers, set out the case against Zalkalns and the most definitive account so far of how Alice died.

Chalmers said a well-preserved cigarette butt found metres from where Alice’s body was recovered contained DNA matching that of Zalkalns.

Her body was found tied in a foetal position, wrapped in bin bags, and had been weighed down with a bike wheel and six sections of a tree trunk in a pyramid formation, he said.

An iPhone cover which was found concealed in Zalkalns’s garden has been identified as belonging to Alice by her sister, the officer said.

Chalmers said there was no evidence that a sexual assault had taken place.

On 4 October, Zalkalns’s body was found in dense woodland in Boston Manor Park, west London, bringing to an end a month-long search.

The Metropolitan police faced some criticism for the length of time it took them to find Alice’s body. On Tuesday they said she was probably killed between the time she disappeared and when her worried parents reported her missing.

Latvian court documents show Zalkalns was found guilty of murdering his wife on 18 June 1998 after a trial in Riga. He was also found guilty of keeping a gun and ammunition without a permit. Zalkalns, who stabbed his wife in remote woodland and buried her in a shallow grave, was sentenced to eight years’ imprisonment.

He is believed to have come to Britain in 2007 and was arrested in 2009 over an alleged indecent assault of a 14-year-old girl in west London. That case was dropped after the girl declined to make a statement.

Zalkalns is believed to have children in Latvia and also to have had a child, now aged one, with a woman in London.