The teen girl found naked and badly beaten in a field has been collected from hospital by the man gardai questioned about the attack.
The 18-year old was stripped and savagely beaten and needed 30 stitches to her head and face.
Gardai believe she could have sustained even more serious injuries if a passerby had not interrupted the attack.
"This man saw her being assaulted and alerted us and a man was arrested at the scene," said a garda source.
There are unconfirmed reports that the teenager was tied up during her horrendous ordeal in a field in Edgeworths-town, Co Longford.
A sexual motive for the attack has been ruled out. The teenager was not raped or sexually assaulted and it is not known why she was naked.
Investigators believe a domestic incident may have spiralled out of control.
The girl was taken to the Midland Regional Hospital in Mullingar before being transferred to Tullamore, Co Offaly, for treatment.
Detectives had been hoping to formally interview her but she discharged herself from the hospital yesterday morning.
It is understood she was collected from the hospital by a close family member and the man suspected of carrying out the attack.
The man, in his 30s and originally from Limerick, was arrested soon after the teenager was found in the field. He was held overnight and questioned under Section 4 of the Criminal Justice Act before being released without charge 12 hours later.
He has convictions for assault, larceny and criminal damage.
The victim has not made a statement and gardai are baffled as to a motive for the attack.
A file is now being prepared for the Director of Public Prosecutions. Yesterday local councillor PJ Reilly told the Herald that the news has left the communities of Edgeworthstown and Granard in shock.
"It's extremely fortunate she was found otherwise you'd never know what would have happened.
"Thankfully she's on the road to recovery."