Shamima Begum and Amira Abase, both 15, and Kadiza Sultana, 16, flew to Turkey from London earlier this week.
The girls are believed to have slipped through the throngs of passengers stranded at Istanbul Ataturk Airport during a record snowfall last week.
With no apparent intelligence request for Turkey to stop the girls, they were able to evade the eyes of the specialized team at the airport trained to stop potential ISIS recruits from moving through Turkey.
The drive to the Syrian border is about 14 hours, but CBC's Nil Koksal reports from Istanbul that the British officers' arrival suggests there is a chance the girls haven't left the city.
Turkish authorities are not revealing any details about how they may be helping British police, but have been vocal in response to criticism that they haven't done enough to seal its 900-kilometre border with Syria.
Turkey says it has increased army units across that border and has already deported more than a thousand people suspected of trying to join ISIS.
Friends, family plead for girls' return
A friend of the girls said that everyone is worried about them and hoped they would come back for their exams in a couple of weeks.
"What were they thinking? We was all shocked when we heard about that. But individually they're all very close they're all just determined to do what they want to do," said Atlanta Broadbent during an interview with Britain's ITV news on Saturday.
Broadbent said they had not talked about going to Syria or Iraq.
"When I saw them a couple of weeks ago, they just seemed normal, how they are every day.... Just going to [study sessions] every day, they seemed normal, they spoke about anything like that," said Broadbent.
Two of the three friends left their east London homes on Tuesday and travelled to Gatwick airport, where they caught a Turkish Airlines flight to Istanbul without telling their families.
'We miss you'
The families of the three girls made emotional appeals for them to return home.
"The message we have for Amira is to get back home. We miss you. We cannot stop crying. Please think twice. Don't go to Syria," Abase Hussen, Amira Abase's father, said to BBC News.
Britain's former foreign secretary, William Hague, said on Sunday that there should be no rush to judge who is responsible.
"Well, there's a responsibility on everybody here, in cases like these: on families, on religious leaders and on security services as well. I'm sure we'll all want to learn lessons from everything that's happened in every case when people go to Syria, but I wouldn't want to rush into any judgments about who's responsible for what," said Hague.
Security forces estimate some 600 British Muslims have travelled to Syria to join the conflict there, some of them with the militant Sunni Islamist group ISIS.
Around half have since returned, and dozens have been arrested in Britain under anti-terrorism legislation.