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."
The girls are among 26 British young women who have gone to join IS fighters in Syria, according to Rowley, who is assistant commissioner of London's Metropolitan Police. 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.
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.
None of the trio passed on the letters, and their relatives have complained that they could have acted to stop the girls going to Syria if they had known the situation.
In her evidence to the committee, Begum's older sister Sahima also said there was no indication that the younger girl had been radicalised.
