The families accused the police on Friday of failing to pass on crucial information that could have stopped the girls’ departure.
The students received letters in relation to a classmate who had fled to Syria last December, but their families never saw the texts as the girls reportedly hid them in their school textbooks.
Shamima Begum, 15, Kadiza Sultana, 16, and Amira Abase, 15, who attended Bethnal Green Academy in east London, were reported missing on February 17.
They flew to the Turkish city of Istanbul from London’s Gatwick Airport on that day and are now believed to be in Syria.
Abase Hussein, the father of Amira, said that he would have intervened if he had seen the police letter, saying, "We would have stopped them. We would have discussed it and taken away their passports from them. This wouldn’t have happened.”
Halima Khanom, sister of Kadiza Sultana, also made similar remarks, stating, "We wouldn’t have been here today doing this if we'd got that letter and known what was going on.”
Keith Vaz, chair of the home affairs committee of MPs, further noted that it was "extraordinary” that the UK police handed the letters to the girls instead of their parents.
"It raises the question of whether the families have been deprived of crucial information they were entitled to receive and added to their anguish,” Vaz said.
The UK government says at least 600 British extremists have joined the ISIL terrorist group.
The Takfiri extremists currently control parts of Syria and Iraq. They have carried out heinous crimes in the two countries, including mass executions and the beheading of people.
The Western states, including Britain, and their allies have long supported the Takfiri groups operating against the government in Syria over the past few years.