Scotland Yard said a 19-year-old man was arrested on Thursday for possession of drugs and conspiracy to murder Sharon Malone, 28. She disappeared in November last year.
Mrs Malone's husband, Gary, and their two children went missing a week ago after he wrote to friends telling them he could no longer cope.
Police confirmed on Thursday they wanted to speak to Mr Malone, 44, about the disappearance of his wife, whom he claimed had vanished when she went to visit a friend.
In one letter Mr Malone said that he might return with sons if his wife came back.
"With every day that passes we are growing increasingly concerned about the safety of Gary and the boys," a police spokesman said earlier on Friday.
Mr Malone and his sons were last seen last Friday when a white Mercedes van he had hired was delivered to his home in Potters Bar, Hertfordshire. He failed to return the van on Monday.
Airports alerted
Airports and ports have been alerted as police try to track him down. Police believe that he might have taken out a substantial sum of money.
Mr Malone's father-in-law Harry Clinch, 60, received one of the letters on Wednesday. It had been posted from the south London area.
"It was just stating that he felt under pressure and he just didn't think he could cope any more." he said.
Detective Chief Inspector Lewis Benjamin said: "I would like to discuss matters surrounding Sharon's disappearance with him."
Police are probing Mr Malone's financial background and making inquiries with friends and family, including his first wife.
Mr Malone is 5ft 10in tall, of medium build, with short dark hair, and sometimes wears dark-rimmed glasses.
Anyone with information should call police on 0181 358 1753.