ISIS-Bound British Schoolgirls: Police Admit Mistakes in Investigation

Scotland Yard admits mistakes during earlier investigation.

Halima Khanon, Sultana's sister, told the BBC that if the family knew about the investigation, they would have "taken precautions with my sister."

"With the benefit of hindsight, we acknowledge that the letters could have been delivered direct to the parents," the police said in a statement.

Surveillance video obtained by the BBC shows the girls waiting in a bus station for about 18 hours. They then boarded a bus to Gaziantep, a Turkish border town that has gained notoriety for being portal for foreign fighters traveling to Syria.

ABC News' Alexander Mallin and The Associated Press contributed to this story.