A white man wearing a burqa has been arrested after the bomb disposal unit detonated a suspicious device in Watford on 18 June.
Hertfordshire Police advised the public to avoid the town centre until further notice, with nearby roads closed off.
Hertfordshire Police said in a statement: "Experts remain at the scene making further assessments and follow up enquiries. This work is likely to continue for some while and as a precaution cordons and road closures in Market Street and the High Street will remain in place."
Army bomb disposal experts and a bomb disposal robot were called to the scene. Hertfordshire Constabulary confirmed that bomb disposal experts had "safely destroyed" the device.
A photograph circulating on social media shows a man wearing a burqa being led away by police.
Michael Dempster from Essex was passing when he saw the arrest taking place.
"I was walking past and I saw a fellow in a burqa having a row with police. There were three officers there, and people from the offices watching.
"You could see enough of his face and from his voice tell that it was a man. but couldn't hear what was being said."
"All the roads were closed, there was traffic backed up and there were police in town."
Police said that the man arrested was carrying two bags with children's books.
Officers have also appealed for information on a woman caught on CCTV walking a dog in the town town centre near the time of the incident to get in touch.
"Police would assure her that she is in no trouble at all and would ask that she make contact with police, via the non-emergency number 101, as soon as possible," said police in a statement.
Earlier police confirmed the arrest.
"Public safety is at the forefront of the police operation and we would ask people to avoid the centre of Watford until further notice," said police.
"As part of the ongoing investigation a man has been arrested in connection with the incident and is being taken into police custody."