Schoolfriends Kadiza Sultana, 16, and 15-year-olds Shamima Begum and Amira Abase, left their homes last month and flew to Istanbul, from where they are believed to have joined Islamic State (IS) jihadists in Syria.
Giving evidence before parliament's home affairs committee, Britain's national police lead on counter-terrorism, Mark Rowley, said the girls paid a travel agent more than 1,000 pounds for their flights.
Asked where they found the money, he said: "We think it's linked to theft from families. We think it's linked to taking jewellery from one of their family members."
But he said that so far "we have no evidence to suggest they are involved in terrorism", and they could return home to Britain without facing charges.
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Earlier, Abase's father Hussen Abase told the committee that his daughter was a victim -- adding that he believed she had been "terrified" by police attention following the flight of another schoolfriend to Syria in December.
Police spoke with Sultana, Begum and Abase and four other girls from the same London school after the 15-year-old girl went missing in December, and gave them a letter requesting further permission from their parents to talk to them.
Abase said of his daughter: "She is the kind of girl who, if it (is) sunset, will call me to pick her up. So how on Earth (can) she travel to abroad and to join ISIS (IS)? The letter terrified my daughter."
He added that the family was "mentally disabled" by her loss, saying: "We focus on our children to come back home, not in prison, because they are victims."
"My sister was into normal teenage things. She used to watch 'Keeping Up With The Kardashians'," she told lawmakers.