More than 50 police officers have been searching for two young children seen on the streets of Nottingham in the early hours of Wednesday.
The first sighting of the boys, aged about four and six and wearing pyjamas, was in Bulwell at 0030 BST.
Police said they had two similar reports at 0530 BST in the same area, where temperatures fell to 1C (34F).
Officers said no children had been reported missing but they remained concerned for the welfare of the boys.
The initial sighting was reported by a member of the public in Cinderhill Road, while the second was near an underpass in Newmarket Road.
Police want to speak with any parent, relative or carer who either knows, or thinks they know, the identity of the boys.
Ch Insp Matt McFarlane said: "Our priority is to establish if these two boys are safe and well and we are continuing to conduct a comprehensive search.
"It is possible they got out in the night and have since returned home. I urge parents to please let us know if these two boys could be their children and to let us know that they are safe and well.
"In addition, any parent, relative or carer, should rest assured they have nothing to fear from reporting their suspicions or concerns to the police."
The children are both described as white with dark hair. The older boy is slim, about 4ft 6ins tall, and was wearing a dark pyjama top.
The second boy is said to be about 4ft tall, slim, and was wearing a blue pyjama top. One of the boys also had a red bag.
Officers have been searching streets, gardens and outbuildings, as well as contacting local schools and talking to teachers, and viewing footage from CCTV cameras in Bulwell.