More than 700 potential terror suspects have traveled to Syria from the U.K. to fight or support extremists, and about half are believed to have returned, British police said Thursday.

Mark Rowley, the national police lead for counter-terrorism, said the suspects were "not aid workers or visiting relatives — they are people of real concern."

Rowley also said Thursday that a record 338 people were detained on suspicion of terrorism offenses since last year. He said more than half of those arrests were related to Syria.

A total of 89 people were convicted of terror-related offenses in that period.

The figures also showed that increasing numbers of women and teenagers with troubled backgrounds were becoming drawn to extremism.

"The make-up of terrorism has changed. Those statistics really illustrate that — you would not have seen that five or 10 years ago," Rowley said. "That mix of families, women, teenagers getting involved in terrorism is something that comes out."

Plans to commit extremist crimes in Britain varied widely in terms of the suspects' capabilities, Rowley said.

While the most sophisticated ones were potentially directed from abroad, Rowley said some inspired by, but not associated with, the Islamic State group often looked to "low-tech" devices such as knives and basic bombs.