Police searching for April Jones have arrested Mark Bridger on suspicion of her murder.
Police confirmed that the five-year-old girl has not been found and the extensive search for her will continue into its fifth day.
Bridger, 46, was originally arrested on suspicion of her abduction. He remains in custody for questioning at Aberystwyth police station.
DS Reg Bevan said: "Mark Bridger has been arrested on suspicion of the murder of April Jones.
"The arrest does not detract from our efforts to find April and we remain committed to finding her."
An appeal for information about Bridger's movements between 6.30pm on Monday and 3.30pm on Tuesday and his Land Rover Discovery with the registration L503 MEP is still in force. Volunteers, however, were being stood down from helping in the search after this "significant development".
Supt Ian John said: "We want to acknowledge the efforts of the community volunteers who have supported the professional searchers in trying to locate her.
"They have been a vital part of our team throughout this search operation.
"The dynamics of the search have now changed and due to the passage of time and the developments within the investigation it is no longer appropriate for us to expect untrained members of the public to continue the search.
"We only require professional searchers to be involved in the ongoing search in and around Machynlleth."
Police have searched Bridger's three-bedroom cottage, outbuildings, and garden in Esgairgeiliog, a few miles north of Machynlleth.
Forensics experts involved in the search were seen moving paving slabs from outside the property.