LONDON (Reuters) - British police on Saturday denied accusations that they had failed to pass on crucial information that could have allowed families to stop three teenage girls from traveling to Syria to join Islamic State militants.

Shamima Begum and Amira Abase, both 15, and Kadiza Sultana, 16, who all attended the same London school, were reported missing on Feb. 17. They flew to Turkey that day and police believe they subsequently made their way to Syria.

Dozens of Britons, mostly young men but also some young women, have joined Islamic State, causing much angst back home about how they were being radicalized and what could be done to stop them.

Police officers had spoken to the girls in the weeks preceding their departure as part of an investigation into the disappearance of one of their school friends, an unnamed 15-year-old girl, who had left for Syria on Dec. 6.

"The teenagers were all being cooperative, they were all being treated as potential witnesses and there was nothing whatsoever to indicate that they themselves were planning to travel to Syria," the police said in a statement.

Several relatives of the girls have complained to British media in the past two days that they had not been told about the first girl who had gone to Syria.

"The police neglected us, the school neglected us. It would have definitely alarmed me ... 100 percent I would have stopped her. They did not warn us, they did not contact us at all," Hussen Abase, father of Amira, told the Guardian newspaper.

Relatives complained in particular that police had handed the girls letters, intended for their parents, requesting that the girls continue to cooperate with the investigation into their friend's departure for Syria.

Instead of handing them to their parents, the three girls hid the letters in school textbooks. The letters were found after they had absconded.

"With the benefit of hindsight, we acknowledge that the letters could have been delivered direct to the parents. However, the parents were already aware ... that Girl 1 had traveled to Syria," the police said.

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They said that after an officer spoke to the girls at their school on Dec. 9, the deputy head teacher contacted their families on police advice to let them know what was going on.

Investigations into the disappearance of all four girls are continuing.

(Reporting by Estelle Shirbon; Editing by Gareth Jones)