Students and staff at a college in West Sussex had to be evacuated following reports of gunshot fire.
An 18-year-old man has been detained following the incident at Crawley College and a firearm and a knife were seized.
Two college staff members sustained minor injuries, which police stressed were not gunshot wounds.
Local officers are liaising with counter-terror police and “all lines of inquiry are open”, police said on Monday evening.
Significant numbers of armed police remained at the scene in the West Sussex town as the area continues to be cordoned off.
Earlier, a group of people were seen being escorted out of a building.
A large police presence – including units from the Metropolitan Police – is in place near the college and the public has been urged to stay away.
Chief Superintendent Howard Hodges said: “We continue to have a large policing presence in Crawley and are appealing for anyone with information to come forward.
“Two staff members at the college suffered minor injuries and presented themselves to a walk-in medical centre.
“No-one else has presented themselves to us with injuries but if you need medical assistance please contact the ambulance service or speak to an officer.
“We are liaising with officers at Counter Terrorism Policing South East (CTPSE). All lines of inquiry are open at this stage.”
Officers received multiple calls shortly after 3.10pm of reports of gunshot fire at the college.
Armed police were dispatched to the scene and an 18-year-old man from Crawley was detained in College Road at 3.20pm, police said.
He remains in custody at this time.
The college tweeted that it was awaiting further information from police but was not aware of any serious injuries.
“Our priority remains the safety of students & staff,” it said.
Anyone with information can contact police by calling 101 or online quoting Operation Reset.