Police have detained a 21-year-old as part of the investigation into missing Glasgow student Karen Buckley.
The man is understood to be Alexander Pacteau, the director of a courier firm, who spent the night with Miss Buckley at his flat after meeting her in a nightclub in Glasgow this weekend.
Police said yesterday (local time) they were now following a "definite line of enquiry".
The probe was stepped up earlier amid growing concerns for the 24-year-old's welfare, who has not been seen for days.
Sniffer dogs, forensic teams and specialist divers scoured areas in the north-east of the city hunting for clues of the student's whereabouts.
Ms Buckley was last seen leaving a nightclub with Mr Pacteau in the early hours of Sunday morning. She reportedly left the property at 4am to walk home.
On Wednesday, officers combed a park in the north-east of the city, Dawsholm Park, where Miss Buckley's black handbag - which she was carrying on the night of her disappearance - was later found by a member of the public. Divers also searched the River Kelvin, which borders the area.
Meanwhile, forensics officers examined the rooms and garden of the flat in Dorchester Avenue in Kelvindale, where Mr Pacteau lives.
Police earlier ruled him out as a suspect but a large-scale search was carried out around his home as officers searched for clues.
A police helicopter also circled between the villages of Milngavie and Drymen to the north of Glasgow, where a grey car linked to the search was seen on Monday.
A Police Scotland spokesman said: "The search resumed at first light this morning with specialist search officers and air support in the area of Dawsholm Park and the north-west of Glasgow."
Mr Pacteau, who police said earlier is not being treated as a suspect, told officers the pair travelled to his flat where the pair were "intimate", according to police.
Miss Buckley left the property at around 4am to return to her home, some four miles (6.4km) across the city, but did not arrive.
Relatives of Mr Pacteau yesterday confirmed 24-year-old Miss Buckley went to his flat from a nightclub before she disappeared.
Mr Pacteau's mother Noreen, 43, told the Daily Record: "She left his property at four o'clock in the morning and she was seen by other witnesses, not Alexander. So he is not a suspect."
Miss Buckley had left The Sanctuary nightclub in the early hours of Sunday morning without collecting her coat. She had told friends she was going to the toilet.
Friends have since said that Miss Buckley had been drinking but was not drunk.
CCTV cameras filmed her talking to a man, believed to be Mr Pacteau, outside the club before getting into a car and travelling to his flat in nearby Kelvindale.
Miss Buckley, who is brunette and 5ft 2in (157cm), was wearing a black jumpsuit and red heels when she was last seen leaving Mr Pacteau's flat in Kelvindale at around 4am.
The family of Miss Buckley, who is from Cork in Ireland, have flown to Scotland to help with the search for the Glasgow Caledonian University student, who studies occupational therapy.
Her mother Marion told a press conference in Glasgow: "We just want Karen home safely, we are desperate. She is our only daughter, we love her dearly.
"If anybody has any information please come forward, we would dearly appreciate it."
Her father John Buckley, 62, added: "We are extremely concerned for her. We are desperate to get her back and safe with her family - she is our only daughter, we love her dearly and just want her to come home safe and sound.
"She always keeps in contact with her family and friends to let them know where she is and what she is up to.
"To not return to her flat or be in contact with her pals - not responding to texts and calls to her mobile is very, very worrying as it is so out of character."
Detective Superintendent Jim Kerr, from Police Scotland's major investigations teams, stressed at a press conference yesterday that the man since identified as Mr Pacteau is "not a suspect".
Mr Kerr said: "We've traced the man she was with in Dorchester Avenue. He believes he was intimate with her at his flat consensually in the early hours of Sunday."
The police officer added: "From what we can see, she does not appear to be under duress, there's no signs of a struggle or reluctance on her part to leave the club."
Police are also keen to find out more about a grey car that was seen on the roads between Milngavie and Drymen north of Glasgow between 11am and 3pm on Monday.
Mr Kerr said: "The activity of this grey car on the Monday is something that we're a wee bit concerned about. This car has been seen at various locations on these roads and I want to know why."
Speaking of Miss Buckley, Mr Kerr added: "We know she left the club with her handbag - which has been found - but left her coat behind and so far has not returned to pick it up.
"Why she left so suddenly without telling her friends we just don't know - it was very much out of character - she is usually very good at keeping in touch with pals."
He added: "We are gravely concerned that Karen has come to some harm, whether that is down to foul play, criminality or she has taken unwell or had an accident is obviously still to be established."
Meanwhile, in Ms Buckley's home in north Cork, a special prayer service was arranged for local people to show support for the family.
The private Mass of Hope, as it was being described, was being held at the Church of Saint Michael The Archangel, Mourneabbey, near the town of Mallow.
Father Joe O'Keeffe, the parish priest, appealed for parishioners and friends of the Buckley family to be given the space to offer prayers for the family.
- Daily Mail