British press reported the body, which has yet to be formally identified, was found in the River Brent and had been carefully hidden.
Commander Graham McNulty, heading the investigation said in a statement Wednesday: "Last night, 30 September, a search was carried out in the River Brent, as part of our ongoing investigation into the disappearance of Alice Gross.
"This is now a murder investigation and I need the public's help to find whoever is responsible."
"Following this search we have sadly recovered a body from the water. This is obviously a significant development and Alice's family has been informed.
"We are unable to make a formal identification at this stage, but clearly this news is devastating for everyone involved in the search for Alice.
"I would urge anyone who may know something to come forward. Even if you have not yet spoken out it is not too late to tell us what you know.
"Our work at this scene is crucial to ensure we capture all the available evidence allowing us to identify who is responsible for this dreadful crime. This may take some time, and I ask people to remain patient with us. I can confirm that significant efforts were made to conceal the body."
14-year-old Alice Gross went missing on August 28. CCTV footage showed Zalkalns in the same area as the teenager at the same time.
Zalkalns served seven years of an eight-year sentence in Latvia for murdering his wife in 1997.
UK police have launched a massive manhunt for Zalkalns, who went to the UK to work as a laborer in 2007. He has not been seen since September.
The search has spread to Latvia, to which it is believed Zalklans may have returned, and Scotland Yard detectives are reportedly in the country being helped by their Latvian counterparts.