Shamima Begum, 15, Kadiza Sultana, 16, and 15-year-old Amira Abase paid more than £1,000 in cash to a travel agent for their flights to Turkey, MPs were told.
Asked how they raised the funds, Metropolitan Police Assistant Commissioner Mark Rowley, the national police lead for counter-terrorism, told the Home Affairs Select Committee: “We think it’s linked to theft from families.
“We think it’s linked to taking jewellery from one of their family members.”
Relatives of the three girls had told the committee there was no sign they had been radicalised and demanded an apology from the police over a letter which was sent home regarding a 15-year-old who had run away to Syria in December.
The parents were angry the letter had been sent home with the girls and not to them directly.
Metropolitan Police Commissioner Sir Bernard Hogan-Howe told the committee: Sir Bernard said: “First of all we’re sorry if the family feel like that, clearly it’s a terrible situation they find themselves in, having lost their daughters in such a horrible way.
“You can only half imagine what a parent is going through at this time. In that sense, I’m sorry they’re in that situation.
“Also sorry the letter we intended to get through, didn’t get through. It’s clear that failed. It was intended for them and failed and for that of course we’re sorry.
“I don’t think we would go as far as saying therefore that caused the girls to go.
“There was nothing more we could have done to prevent that. Because at the beginning we were trying to get from these girls information about a further young woman who had actually left in December that was our principle reason for talking to that family.”
Mr Hogan-Howe said neither the families, police or school were aware of the three teenagers intentions.
Foreign Secretary Philip Hammond yesterday mounted a fierce defence of the work of the intelligence agencies and described their critics as apologists for terrorism.
His comments appeared to be directed at critics like Asim Qureshi of the campaign group Cage, who said it was MI5’s attempts to recruit Mohammed Emwazi - recently unmasked in reports as Jihadi John - as an informer that led to him becoming radicalised.
Mr Hammond used the same speech to warn of the potential threat posed by Russia and the destabilising effect its actions are havving on Eastern Europe.
In some of the strongest comments by ministers since the crisis in Ukraine, Mr Hammond said Russian leader Vladimir Putin had decisively rejected efforts to draw his country into a “rules-based” international order and was now actively seeking to subvert it.
He said that in the face of the “increasingly aggressive” stance of the Russian military, the effort to establish its intentions was now once again a “vital” element of the work of Britain’s intelligence agencies - MI5, MI6 and GCHQ.
“We are in familiar territory for anyone over the age of about 50, with Russia’s behaviour a stark reminder that it has the potential to pose the single greatest threat to our security.
“Hence, continuing to gather intelligence on Russia’s capabilities and intentions will remain a vital part of intelligence effort for the foreseeable future,” he said.