The man detained on Thursday evening is suspected of having fired the shots that left the two victims needing hospital treatment, Expressen reports.
He is a 41-year-old bouncer with an address in the prosperous Stockholm suburb of Lidingö, according to the newspaper.
“A detention order was put out for this person during the day and the person was brought in for questioning this evening,” police spokeswoman Petra Sjölander told TT late on Thursday.
The injured man’s ex-wife, a woman in her forties, was arrested shortly after the attack on suspicions that she hired a gunman in an attempt to have her former husband and his new girlfriend killed. She has denied any involvement in the attack.
“She rejects all accusations and was not involved in what happened in any way,” said her lawyer, Claes Borgström.
Both victims are described as being in a stable condition at Karolinska University Hospital.
The violent drama unfolded outside the Storkyrka School in Stockholm’s Old Town around 5.30pm on Tuesday.
The victims, a man and a woman both in their thirties, were hit by several bullets. The man was shot in the chest and head, while the woman was hit in the throat and face.
Police said the assailant arrived at the scene of the shooting at around 4pm on Tuesday and waited for the aristocrat and his girlfriend to arrive.
The shooter then fled the scene on a yellow scooter wearing a helmet with a visor pulled down over his face.