Between 30 and 40 convicted Latvians living in the UK are being supervised by probation officers in their native country via email, according to a criminal justice expert.
The revelation comes after forensic searches were conducted at the home of a Latvian builder named as the prime suspect in the search for Alice Gross, the missing London schoolgirl.
Scotland Yard said it was looking for Arnis Zalkalns, 41, after he was identified on CCTV near the towpath in west London where Alice went missing. Latvian police confirmed Zalkalns served a custodial sentence after he was convicted of bludgeoning his wife, Rudite, to death in his home country in 1998.
He served seven years in prison, prompting questions over why he was allowed into the UK and how much the authorities knew of his violent past.
Harry Fletcher, a former assistant general secretary at the probation union, Napo, and now director of the Digital-Trust, a charity that campaigns against online abuse, said that Latvian offenders living in the UK were obliged only to email their probation officers once a fortnight.
The revelation will further intensify the focus on the supervision of offenders from EU member states residing in the UK. British citizens who had been convicted of murder would be subject to stringent supervision orders for the rest of their lives and details of their offences would be lodged with the relevant authorities.
They would have to be seen regularly by a probation officer and would have to report any changes in their lives. They would also have to seek permission to travel abroad. But Latvia does not insist on such oversight, Fletcher claimed.
"The Latvian authorities don't have a postal address for them, just email addresses," Fletcher said.
"These are offenders currently on licence, some for serious convictions. The Latvian authorities can't take their passports away because of their right to free movement. But this is a legal loophole that needs closing."
Zalkalns went missing a week after Alice was last seen on 28 August, when she was spotted on CCTV by the Grand Union canal in west London. Zalkalns's bicycle was discovered on Friday, although police declined to say where.
It emerged last week that Zalkalns had been arrested on suspicion of indecent assault on a 14-year-old girl in 2009, but no further action was taken.
The mother of his murdered wife described her former son-in-law as a "control freak with a fierce temper".
Scotland Yard insisted it has no evidence suggesting Alice, 14, who suffered from anorexia, has come to harm.
"This is not a murder inquiry in the sense that we don't have any evidence or information to say that Alice is not alive," said Det Supt Carl Mehta, from the Met's homicide and major crime command.
The Home Office can stop EU citizens entering the UK if they are deemed to be a threat to public security, public safety or public health. Each case is looked at individually.
But Scotland Yard said there was no record in the UK of Zalkalns's murder conviction, raising questions about the nature of information sharing between EU member states regarding convicted criminals and the ability of the Home Office to assess how much of a threat they pose.
It is unclear whether the Home Office was aware Zalkalns was a murderer. It has a longstanding practice of not commenting on individual cases.
"We have detailed arrangements in place to identify people of concern entering the UK," said a Home Office spokesman.
"All passengers are checked against police, security and immigration watchlists and where we are aware of individuals who pose a risk, Border Force officers can – and do – refuse them entry."
A reward of up to £20,000 is being offered for anyone who has information that leads detectives to find Alice.
Zalkalns is described as white, 5'10", of stocky build and with dark brown hair that he normally wears tied in a pony tail.
Police have said that Zalkalns "potentially poses a risk to the public" and have asked anyone who sees him not to approach him and to dial 999