UK police have been "complicit" in the alleged "grooming and radicalisation" of three Bradford sisters believed to have travelled to Syria with their children, their families' solicitors say.
The two women disappeared with a third sister, Zohra Dawood, and their nine children, aged between three and 15, following a trip to Saudi Arabia.
The letter claimed both men had "grave concerns" about the way that West Yorkshire Police and its North East Counter Terrorism Unit (Nectu) had handled the matter.
It alleged that police had been "actively promoting and encouraging contact with the brother-in-law of our clients, whom, it is believed, is fighting in Syria".
"It would appear that there has been a reckless disregard as to the consequences of any such contact on the families of those whom we represent."
Plainly, by the NECTU allowing this contact they ave been complicit in the grooming and radicalising of the women. If this contact between them had been prevented or clients would not now be facing such circumstances.
The letter also claimed that the two men had made "repeated attempts" to get updates on the progress of the investigation, claiming that these were met by a "wholesale lack of information".
"Instead, the police have been anxious to ensure that there is no criticism of them and their tactics in allowing, encouraging and promoting contact with someone believed to be in Syria," it added.
Following the letter, Vaz told the Mail on Sunday: "That three women could disappear from the UK to take nine children into the heart of a war zone is incomprehensible.
"The claims of their relatives in the UK that their links with IS were 'encouraged' by the authorities is concerning."
West Yorkshire Police could not be immediately reached for comment, but told the paper it completely rejected accusations.