A former detective who once interrogated Arnis Zalkalns, the chief suspect in the case of missing teenager Alice Gross, described him as a "callous" murderer who could "kill again".
The retired police officer, who did not want to be named, interviewed Zalkalns following the brutal murder of his wife Rudite in 1998.
"I've investigated many murder cases in my career but this was the most callous case I've ever come across," he said.
"This man made me sick. In interview, sometimes he cried, sometimes he got agitated and sometimes he had violent mood swings. But he never showed any remorse. He always said: 'She deserved it'."
"He is the type of person who could kill again. This man has come back to haunt us."
His comments come as Met Police has formally requested the case file of Zalkaln's murder conviction in which he served seven years for stabbing his wife Rudite to death in his native country.
The Met has been criticised for waiting four days before contacting its Latvian counterpart for help in tracing Zalkalns.
Their search operation for the 14-year-old missing girl has been described as the largest since the 7/7 bombings.
But a police source in Riga reportedly told The Times on Sunday that the Met had delayed before asking Latvia for assistance.
"There is a possibility he might have fled here so we are doing everything we can to help. Of course they can ask unofficially and of course we are happy to look for him," the source added.
Alice Gross and Zalkalns both disappeared on 28 August. CCTV footage showed Gross walking down the same towpath in southwest London as Zalkalns who was captured moments after cycling on a bike.
A Met spokesman said the force was "maintaining a close and productive relationship" with Latvian authorities.
He said: "We are not prepared to elaborate further on the nature of our information sharing with them. We are not prepared to give a running commentary on the specifics of our active investigation."