The families of three British students, who reportedly were trying to cross into Syria to join Daesh, accuse police of not having informed them of an "important letter."

Recently, Shamima Begum, 15, Amira Abase, 15, and Kadiza Sultana, 16, left Bethnal Green Academy in east London in February for Turkey, reportedly to cross into Syria to join extremists.

Istanbul police's surveillance video showed the trio at a major bus station in the city on Feb. 17. A British police team arrived in Turkey on Feb. 23 to assist in the investigations.

The families now accuse police of negligence, as they did not inform them about a letter written given to the girls to be submitted to their parents.

In the letter, dated Feb. 2, police asked the families to allow their children to give a deposition after one of their friends left the country in 2014 to join Daesh.

"I understand your daughter may have known the missing 15-year-old as a friend. It is my role to understand the reasons why she has decided to leave this country. I am trying to gather information, which may help to find her and reunite her with her family. For these reasons, I seek your permission to speak to your daughter," the letter said.

However, the girls kept the letters hidden in their bedrooms.

The families say that, had they known about the letter, they might have been able to prevent the girls from leaving the country.

"I would have seized my daughter's passport if I had known about the letter," said Hussein, Amira Abase's father.

"It is incredible that the school and the police did not inform the families," said Fahmida Aziz, Kadiza Sultana's cousin.