There were reportedly concerns about the behaviour of a British man in the months before he was arrested for allegedly trying to cross illegally into Syria.
Waheed Ahmed, 21, was among nine members of the same family who were detained in Turkey on Wednesday and are set to be deported.
British police have been searching the home of his father Shakil Ahmed, who is a Labour councillor in Rochdale.
They have also searched four other properties - two in Heywood and two in Rochdale. Two of the searches are ongoing.
Mohammed Shafiq, who is a friend of Shakil Ahmed, said of Waheed: "There were concerns in the last six months to a year about a change in his behaviour.
"And a change in his attitude towards various different issues."
He also told Sky News: "That was causing concern for people in the community and his family."
Waheed Ahmed was with his aunt, two cousins and the wife of one of his cousins when they were stopped by authorities in southern Turkey, near the Syrian border. They had also taken four children with them.
The 21-year-old is due to be sent back to the UK along with Zareeda Bi, 47, Maboob Yasin, 22, Habib Yasin, 24, and Samia Bi, 22, and the youngsters aged one, three, eight and 11.
They are understood to have been kept in a police station in Turkey on Thursday night.
A spokesperson for Greater Manchester Police said: "It remains the case that we are not aware of any time scale for the potential return of the nine individuals."
Most of the group flew from Manchester on 27 March and Waheed Ahmed joined them three days later on a flight from Birmingham.
Shakil Ahmed, who represents the Kingsway ward on Rochdale Borough Council, said the group were on holiday but he had thought his son was on a work placement in Birmingham.
"My son is a good Muslim and his loyalties belong to Britain, so I don't understand what he's doing there.
"If I thought for a second that he was in danger of being radicalised, I would have reported him to the authorities.
"I just want to speak to my son and get him home as soon as possible so I can find out what's going on."
Usman Nawaz, who went to the same school as Waheed Ahmed in Rochdale, said mosques and his school in Rochdale were not to blame for Waheed's actions.
The 25-year-old, a former member of the Young Muslims' Advisory Group and mentor to young Muslims through Rochdale's Youth Council, said: "For some it's an adventure but for some they think that they are doing something noble.
"The ideology which is peddled, this very hardcore understanding of the faith, one which is state sponsored by Saudi Arabia, that has to be challenged and it's quite difficult to challenge it in a coherent manner when the Saudi ideology has the backing of petrodollars."
A friend of Waheed Ahmed believes he could have travelled for humanitarian reasons.
Bassat Yussu said: "We used to talk about Palestine and Syria and what was going on around there and sometimes he used to collect charity money."
Neighbour Mohammed Sharif said he "never had suspicion about the family" and remembered Waheed as a "nice quiet young lad" who would "speak to you with respect".
Officers are trying to establish their reason for travelling to the Syrian border.
Assistant Chief Constable Ian Wiggett said: "What is obviously concerning is why a family were seemingly attempting to take very young and vulnerable children into a war zone.
"Such a volatile and dangerous environment is no place for them whatsoever."
The Metropolitan Police believe around 600 Britons have travelled to Syria and Iraq since the conflict began, while around half are thought to have returned to the UK.
