Police were "complicit in the grooming and radicalising" of three sisters who are feared to have travelled to Syria with their nine children, according to lawyers representing two of the youngsters' fathers.
British sisters Khadija Dawood, 30, Sugra Dawood, 34, and Zohra Dawood, 33, and their children, aged between three and 15, are feared to have travelled to link up with terror group Islamic State in the country.
Last week, Akhtar Iqbal and Mohammed Shoaib broke down as they pleaded desperately for their wives to return so they can go back to their "normal lives".
Now solicitors representing the two men have written to the chair of the Commons Home Affairs Committee, Keith Vaz, saying their clients are "extremely disappointed with the manner in which this matter is being investigated by West Yorkshire Police and have expressed grave concerns in the conduct of the police leading to these events".
Referring to a brother of the sisters, who is believed to have joined Islamic State, the letter from Khan Solicitors, in Bradford, said: "Indeed, we are alarmed by the fact that the police have been actively promoting and encouraging contact with the brother-in-law of our client 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."
The letter, posted by Labour MP Mr Vaz on his website, continued: "Plainly, by the NECTU (North East Counter Terrorism Unit) allowing this contact they have been complicit in the grooming and radicalising of the women. If this contact between them had been prevented our clients would not now be facing such circumstances."
The solicitors explained how their clients have not had the information they want about the progress of the investigation and accused the police of making sure they are not criticised for their tactic of "allowing, encouraging and promoting contact with somebody believed to be in Syria".
The letter said: "The actions and misjudgement of the NECTU has placed the lives of 12 British citizens at risk, nine of which are innocent children, the youngest being three years of age."
The letter also confirms that Zohra Dawood left a voice message for her family on Wednesday indicating she was already in Syria and that all the missing people had all travelled there together "due to the oppressive nature of the continued surveillance by the police".
It said the police released this information last week without any consultation with the fathers and that senior officers have refused to meet the two men.
The missing group travelled to Saudi Arabia on May 28 to go on an Islamic pilgrimage.
They were due to return to the UK on June 11, but broke off all contact with family in Britain two days earlier on June 9.
It is believed they boarded a flight to Turkey that day and now have crossed into Syria.
It later emerged that the same group tried to fly earlier this year. The BBC reported that the sisters and their children, from Bradford, were originally booked to travel from Manchester to Mecca and Medina on March 19.
The North East Counter Terrorism Unit confirmed that the same group had been stopped and made the subject of security checks earlier this year.
Mr Iqbal and Mr Shoaib broke down in tears as they pleaded with their wives to come home at a press conference organised by the family last week.
Mr Iqbal's wife Sugra has gone away with his five children - Ismaeel, three, Mariya, five, Zaynab, eight, Ibrahim, 14, and 15-year-old Junaid Ahmed.
Appealing directly to his family, he said: "Please, please call me. It's been eight, nine days, you are out and we don't know where you are.
"I miss you, I love you. All of you, I love you a lot. I can't live without you.
"To my family, please, please call me (so) at least I know where you are. Are you safe?"
Mr Shoaib's wife Khadija has disappeared with his two children - five-year-old Muhammad Haseeb and Maryam Siddiqui, seven.
He said: "Please come back home with the kids, I know the kids can't live without me and you. Please bring them home, they can't live without me."
He added: "I'm not angry, please come back, everything is normal, come back to normal life, please."
Zohra's husband is not currently living in the UK.
Later, Mr Vaz issued a statement saying he will meet with the two fathers tomorrow.
West Yorkshire Assistant Chief Constable Russ Foster said: "We have not seen the letter so cannot comment in detail on its contents. However, we completely reject accusations that the police were complicit in the alleged grooming of the missing family or that we were oppressive to them.
"While we do not comment on all aspects of police work for valid operational and safeguarding reasons, this is an ongoing investigation and we are continuing to do everything we can to find the missing family and to ensure the safety of the children"
"Their relatives have been kept informed throughout this investigation and we are pleased they expressed their satisfaction with the support they have received so far. This will continue."