Authorities are continuing to search for three London teenagers who are believed to be trying to join the Islamic State in Iraq and Syria extremist group, as more details about the girls emerge.

The three missing teens — Shamima Begum, Amira Abase, both 15, and Kadiza Sultana, 16 — flew to Istanbul last week. They are believed to be attempting to join ISIS fighters in Syria.

All three attended Bethnal Green Academy in east London.

The school's principal said police interviewed the teenagers after a 15-year-old friend of theirs travelled to Syria in December, according to the Guardian.

"The police spoke to that student's friends at the time and, further to this, they indicated that there was no evidence that the girls were at risk of being radicalized or absconding," Mark Keary said in a statement.

Shamima Begum, 15

There was no indication that Shamima Begum planned to leave and she appeared normal in the lead-up to her disappearance, according to her older sister.

"You're not in any trouble here, we all love you," Renu Begum said. "If anybody's convinced you of anything, they're wrong."

Renu Begum said she hoped her sister travelled to Syria to bring back the other teen who left for Syria in December, according to BBC.

Shamima used the passport of her 17-year-old sister Aklima to travel to Istanbul on Tuesday, BBC also reported

Sky News reported that Shamima had contact with Aqsa Mahmood, who travelled from Glasgow to Syria to marry an extremist in 2013. The teenager tweeted to Mahmood two days before she disappeared asking that they exchange private messages.

Amira Abase, 15

Amira's father said she appeared normal and there were no signs that she was acting differently before her disappearance.

Abase Hussen told BBC News that his daughter said goodbye in a normal way before her disappearance on Tuesday, saying that she was in a hurry when she left.

Sky News reported that she said she was going to a wedding. Her family contacted police around midnight when she failed to return home.

Hussen also said his daughter didn't express an interest in Syria and did not talk about international affairs, including the politics of the Middle East, ahead of her disappearance.

"We miss you. We cannot stop crying," he said in a video posted on BBC News. "Please think twice. Don't go to Syria."

Kadiza Sultana, 16

Kadiza Sultana's older sister told BBC that her family is hurting and is not sure if the teen is safe.

"Find the courage in your heart to contact us and let us know that you are safe and you are OK," Halima Khanom said.