Three young British men who were stopped from travelling to Syria from Turkey and arrested have been released on bail, the Metropolitan Police says.
The teenagers, two aged 17 and one 19, from north-west London were flown back to the UK on Saturday night.
They were arrested on suspicion of preparing terrorist acts and have been bailed pending further inquiries.
Meanwhile, an 18-year-old man is being held on suspicion of preparing to travel to Syria to join IS.
The man, from Hodge Hill in Birmingham, was detained on Monday morning in a pre-planned and intelligence-led operation, West Midlands Police said.
The three Britons stopped in Turkey were apprehended after UK police alerted Turkish officials after a tip-off from the younger teenagers' parents, the Times has said.
Counter-terrorism officers were initially made aware that the two 17-year-olds had gone missing and were believed to be travelling to Syria on Friday.
Their parents contacted police when they did not return home after Friday prayers, according to the Times.
After meeting the boys' families Keith Vaz MP said: The families of the two of the young men that I met are honourable, decent, hard-working British citizens who knew absolutely nothing about their sons' decisions.
"They acted swiftly, with the support of the leader of the council to alert the police."
Further enquiries revealed the pair had travelled with a third man, police said.
'Security co-operation'
"Officers alerted the Turkish authorities who were able to intercept all three males, preventing travel to Syria," a police spokesman added.
They were returned to the UK at about 23:10 GMT on Saturday and were arrested by counter-terrorism officers.
They have been bailed to return to a central London police station pending further enquiries.
The trio had flown to Istanbul from Barcelona in Spain, a Turkish official told the BBC.
The two 17-year-olds were stopped at Istanbul's Sabiha Gokcen airport by Turkish authorities acting on intelligence provided by British police.
Kadiza Sultana, Amira Abase and Shamima Begum (L-R) left Britain in February
However, the 19-year-old man was only detained after being questioned by Turkish police, the official said. He also was arrested at the airport.
"This is a good and a clear example of how the security co-operation between Western intelligence agencies and Turkey should work," the official added.
BBC correspondent Andy Moore said the development came after "recriminations" between UK police and Turkish officials following the disappearance of three girls who attended Bethnal Green Academy i east London.
"On this occasion it seems that the warning was raised in the UK and that was communicated very quickly to Turkey," said our correspondent.
The disappearance of the three girls led to criticisms from Turkey's deputy prime minister, who said Turkish officials had not been given enough warning about their disappearance.
Shamima Begum, Amira Abase, both 15, and Kadiza Sultana, 16 - all from East London - took flights to Istanbul last month, from where it is feared they travelled to join IS militants in Syria.
The parents of a so-called "jihadi bride" from Scotland say their daughter denies recruiting the girls to the IS cause.
Khalida and Muzaffar Mahmood said they had been in touch with 20-year-old Aqsa Mahmood last week, and she insisted she had not been in touch with the Bethnal Green girls.
'Need for vigilance'
Mr Vaz, who chairs the Home Affairs Select Committee, said the police and Turkish security agencies should be commended for the speed in which they acted.
He said: "I have spoken to the Turkish ambassador to congratulate him on the action taken by the Turkish authorities.
"This is a rare good news story in the fight against terrorism and is a lesson to us all that we need to be vigilant against those who seek to groom our young men and women in this way.
"Clearly this flight of young people to Turkey in order to go to Syria is on a much larger scale than we envisaged."
"We need to prevent people going in the first place and that is why parents need to be vigilant but we also need co-operation from the Turkish authorities in order to stop them from going further.
A senior Turkish government official told the BBC that Turkish security agencies have drawn up a "no-entry" list of 12,500 people, with some volunteers being as young as 14.
They have also deported more than 1,100 people suspected of wanting to join Islamic State (IS).
Speaking on condition of anonymity, he complained that Western intelligence agencies are not doing enough to prevent the would-be fighters leaving their countries of origin.
IS fighters in Raqqa
About 600 Britons are believed to have travelled to Syria and Iraq since the conflict began, according to Metropolitan Police Assistant Commissioner Mark Rowley.
The Home Office estimates around half of those have returned.
The BBC understands more than 500 Britons are believed to have travelled to the region to join militant organisations, while 100 Western volunteers, including some from the UK, went to fight with the Kurdish forces against IS.
Jamshed Javeed, a teacher from Bolton who admitted Syria-related terror offences, was jailed for six years earlier this month.
Armed police at Heathrow
Last year there were 327 terrorism-related arrests, more than half of them related to Syria, leading to 64 charges and prosecutions.
These numbers - and regular reports that some young people are still trying to get to Syria - are evidence enough that the problem isn't going to be solved by arrests alone.
The police will arrest and charge if there is clear evidence that someone has committed a terrorist offence.
But they will also look at other tactics to disrupt someone's intentions. Their counter-terrorism toolbox now includes beefed-up powers to stop people at ports, seize passports and place the most dangerous suspects under near-constant monitoring.
But most importantly, the recently passed Counter-Terrorism and Security Act places a legal duty on public authorities to start doing more to combat extremism.
The police will continue to make arrests - but the focus is increasingly moving to the really difficult business of preventing someone being radicalised in the first place.
The news comes as the National Police Counter Terrorism Network and partners have rolled out an advertising campaign designed to reach out to families, to prevent young people travelling to Syria.
It will involve adverts appearing in minority ethnic media across the country.
The awareness campaign features the relationship between a mother and daughter and encourages parents to discuss issues such as travelling to Syria and what they are viewing online.
In the last year 22 women and girls have been reported missing by families who feared they had travelled to Syria.
Kalsoom Bashir of Inspire and Deputy Assistant Commissioner Helen Ball
Deputy Assistant Commissioner Helen Ball, counter-terrorism co-ordinator, said police were increasingly concerned about the numbers of young women who have travelled or are intending to travel to Syria.
She said: "It is an extremely dangerous place and the reality of the lifestyle they are greeted with when they arrive is far from that promoted online by terrorist groups.
"The option of returning home is often taken away from them, leaving families at home devastated and with very few options to secure a safe return for their loved one.
"We want to increase families their confidence in the police and partners to encourage them to come forward at the earliest opportunity so that we can intervene and help."
Kalsoom Bashir from the organisation Inspire, which works with Muslim women to tackle extremism, said: "Having seen the devastation facing families where a loved one has travelled to Syria, I would advise families to keep their children close, to constantly remind them that they are loved, that they are part of a strong family network and that they can talk to you about anything they are worried about."
