Three missing Bradford sisters feared to be trying to get to Syria with their nine children were stopped before an earlier flight, the BBC understands.
Sources said Khadija, Sugra and Zohra Dawood and their children missed a flight to Saudi Arabia in March after being questioned by security officials.
However, they were subsequently cleared to travel and rebooked their flights.
Earlier, police revealed one of the women has "made contact" with family, indicating she may already be in Syria.
The BBC understands middle sister Zohra Dawood, 33, got in touch with relatives in the UK to say she was in Syria.
The women's brother, Ahmed Dawood, is believed to be fighting with extremists in the country, parts of which are controlled by Islamic State militants.
The Dawood sisters and their children were originally booked to fly from Manchester to Jeddah for a religious pilgrimage on 19 March.
The flights were arranged via a Bradford travel agent and a company in London. The BBC has seen documents confirming the intended travel.
However, the extended family were stopped by security officials and questioned, causing them to miss their flight.
Police have special powers at airports to stop and question travellers on suspicion of involvement in terrorism.
- Ismaeel Iqbal, three
- Mariya Iqbal, five
- Muhammad Haseeb, five
- Nurah Binte Zubair, five
- Maryam Siddiqui, seven
- Haafiyah Binte Zubair, eight
- Zaynab Iqbal, eight
- Ibrahim Iqbal, 14
- Junaid Ahmed Iqbal, 15
- Khadija Dawood, 30
- Sugra Dawood, 34
- Zohra Dawood, 33
The BBC has also spoken to the taxi firm which collected the party from the airport in March, who confirmed they were told the women and children returned home without catching the flight.
Days later, the family rebooked flights, telling their travel agent they had cleared the matter up after discussions with the police.
However, because of the size of the party and the Easter holidays, the women were unable to book another departure date until May.
They eventually left Manchester Airport on 28 May and were due to stay in Saudi Arabia until 11 June. However, they left on 9 June and took a flight to Istanbul instead.
The BBC understands they told the manager of their accommodation on 9 June that they intended to travel to Mecca before returning to the UK, and left taking the keys to their rooms.
In a statement to the BBC, the North East Counter Terrorism Unit said: "The North East CTU can confirm that the same group were stopped and subject of security checks earlier this year.
"As a result, enquiries revealed that the visit was a religious pilgrimage to Saudi Arabia and the group were not prevented from travelling. However the process of conducting those enquiries caused the group to miss the flight.
"On a daily basis a number of people are subject to security checks prior to boarding flights.
"It is worth repeating that the party were not legally prevented from travelling."
'Smuggled for £5'
by BBC Middle East correspondent Paul Wood, in Turkey
Smugglers on the border told us the jihadis had one illegal border crossing still working. A smuggler called Abu Ahmed said he guided people in through the wire for the equivalent of £5 each.
Sometimes that price goes up when there are more guards on the border, he told me. But it's not difficult, you just wait for a shift change. You still have to pay a bribe. The Turks take a cut.
It's easy then for people heading to the so-called caliphate to get in. It is difficult and dangerous to get out.
A Syrian activist told me there were as many as 200 women - widows of jihadis or those not yet married - under house arrest in the heart of Islamic State territory.
The Bradford families may find they have bought a one-way ticket.
The BBC's security correspondent Gordon Corera said there were questions about how strong initial suspicions were, and why the group were able to travel a few months later.
"However, from a police point of view, it is difficult. They have limited powers, and they would not want to be seen to harass people going on a religious pilgrimage," he said.
West Yorkshire Police says it is "extremely concerned" for the group's safety after receiving information that at least one of the women may have crossed the border into Syria - parts of which are controlled by Islamic State militants.
Timeline of the disappearance
- Pre-June 2014: The sisters' brother travels to Syria to fight with extremists
- 19 March 2015: Sisters attempt to leave the UK but are questioned by security officials and miss their flight
- 28 May: Family travel from Bradford to Saudi Arabian city of Medina on pilgrimage
- 8 June: Fathers' last conversation with their children
- 9 June: Mothers and seven of the nine children thought to have boarded flight to Istanbul in Turkey - a commonly-used route into Syria
- 11 June: Family had been expected to return to UK. Their disappearance is reported to the police
- 17 June: Police say one of the women "made contact" with their family, and there are indications the group may have crossed the border into Syria
Assistant Chief Constable Russ Foster said: "We have received information that contact has been made with the family in the UK which suggests one of the missing adults may be in Syria.
"Contact has been made by one of the missing women and there is an indication that they may have already crossed the border into Syria but this is uncorroborated."
Metropolitan Police Commissioner Sir Bernard Hogan-Howe said it was "almost impossible to believe" there were "mums who've taken their children to a war zone".
It has also emerged that the mothers had no authorisation from education authorities to take their children out of school prior to the trip.
Talwinder Bhambra, acting headteacher of Marshfield Primary School, which some of the children attended, said: "We are obviously deeply concerned about the missing children and we are doing everything we can to assist the police and to support the affected families."
Meanwhile, the estranged husband of one of the sisters spoke publicly for the first time.
Syed Zubair Ahmed, the estranged husband of Zohra Dawood and father of two of the missing children, told the BBC from Pakistan he was "shocked" by the news of his wife's disappearance.
He said he left the UK seven months ago after his wife "shunned" him, and had not spoken to her since.
He had not had any indication his wife might be considering attempting to travel to Syria, he said, and her brother had seemed like a "nice guy". He also spoke of his hopes that the UK government would do all it could to bring the family back home.
The husbands of the other two sisters made an emotional appeal on Tuesday, saying they "miss and love" them.
They said they last spoke to their children on 8 June.
West Yorkshire Police has been working with the extended Dawood family and the Turkish authorities to try to establish the group's whereabouts.
They said anyone who was concerned that a family member may be contemplating travelling to Syria should contact police and partners via the free phone Anti-Terrorist Hotline number on 0800 789 321.
- The Britons going to Syria and Iraq: The stories of those who have died, been convicted of offences relating to the Islamic State conflict or are still in Syria or Iraq
- Prevent strategy: BBC security correspondent Frank Gardner asks if it is failing to stop radicalisation.