Britons who return after fighting alongside Islamic State should be regarded as "potentially very dangerous", a former terror laws watchdog has said.
David Anderson QC suggested security agencies should take a "sceptical" approach towards UK nationals who have been living in territory controlled by the group for several years.
His remarks come amid fierce debate over how battle-hardened extremists should be managed if they seek to come back.
Mr Anderson told MPs that where possible returnees should be prosecuted - but noted there are difficulties securing convictions for offences committed in Syria.
On those who cannot be pursued through the courts, the barrister said: "All these people need to be looked at very carefully indeed.
"My first reaction would not be to look at them in a particularly generous spirit.
"I think the starting point has to be that these are potentially very dangerous people."
The Government estimates that approximately 850 UK-linked individuals "of national security concern" have travelled to engage with the Syrian conflict.
Just under half, or around 400, have returned, while 15% are believed to have died.
Mr Anderson, who was Independent Reviewer of Terrorism Legislation from 2011 to 2017 , suggested authorities were not currently dealing with a major influx of returnees but he noted that a number have come back in the past.
He told the Commons Home Affairs Committee:
"If you talk to the security services what they will tend to tell you is the ones who have come back tend to be the ones who went in the early days.
"Maybe they went out there as charitable workers and then they stayed, or they got in with the wrong crowd, or they went out of curiosity, or they went to get married to somebody or to live in an ideal state.
"Some of those people were disillusioned and do need to be treated sensitively when they come back.
"The ones who stayed out there past the beheadings of 2014, past the battles and so on, one has to be quite sceptical.
"That's not to say they are all evil, human beings are very complicated."
For cases where prosecution is not possible, Mr Anderson said authorities have a number of options including terrorism prevention orders, surveillance and the Prevent anti-extremism programme.
Questions over how returning jihadis are managed by the counter-terrorism agencies have intensified as IS comes under fierce military pressure.
Last year Mr Anderson's successor as Independent Reviewer Max Hill QC called for a focus on "reintegration" in cases where authorities have decided individuals who return should not face prosecution.
Mr Hill said it was right that security services have left space for those who travelled out of a sense of naivety, at a young age and who return in a "state of utter disillusionment" to be diverted away from the criminal courts.
In December Defence Secretary Gavin Williamson argued that everything should be done to "destroy and eliminate" the threat from returning fighters, adding: "A dead terrorist can't cause any harm to Britain."
Elsewhere in a wide-ranging evidence session, Mr Anderson told the committee he sensed that technology companies are "doing more than they did" to tackle terrorist or extremist content online.
But he added: "There are whole areas where they are not doing anything."