Attorney Bob Massi said he was with family members of Pamela Vitale when they were notified that an arrest was made in her killing.
A 16-year-old boy who lived near Vitale and Horowitz's hilltop estate was taken into custody, according to the San Francisco Chronicle and The Contra Costa Times, which cited law enforcement sources on their Web sites.
Contra Costa County Sheriff's Department spokesman Jimmy Lee would not confirm an arrest had been made, but a news conference was scheduled for late Thursday morning.
Usually, Horowitz, a celebrity defense lawyer, analyzes criminal cases on television news shows.
But, says CBS News correspondent John Blackstone, the murder of his wife on their estate north of San Francisco had turned the television tables on Horowitz, focusing the attention of his fellow TV lawyers on analyzing a case involving his family.
Horowitz found wife Pamela Vitale's body Saturday at the trailer home they were staying in while their dream house was finished elsewhere on the estate. Authorities say she was beaten to death.
Police questioned dozens of people, including Horowitz. He returned to his home Wednesday to collect photos for Vitale's funeral Thursday.
Horowitz told reporters Wednesday that he somehow found it comforting, visiting the place where Vitale died.
"Even though it was horrible all the other times I was there," he said, "this time, I just sort of felt … You could almost smell us in there, like you're really there with the person you love."
The murder has made Horowitz the subject of the sort of case he'd dissect as a legal analyst on television. "I think we have to find the right person, not just any person, so I am not going to comment on police work," he told reporters.
Still, Blackstone said, Horowitz has returned to the broadcasts on which he frequently appeared.
On MSNBC's "Abrams Report," Horowitz said: "This is turning into, and it's going to be, a media event. This is my life, and me and Pamela chose to make me a media person but we didn't choose for Pamela to be murdered."
It's created a curious situation, Blackstone said — the legal analyst being analyzed.
Blackstone asked legal analyst Robert Talbot what Horowitz would say about a murder victim's husband who goes on television at this point in an investigation.
"I would say," Talbot responded, "that Daniel would say, 'Just don't do it. No matter how innocent you are, you don't want to say anything that might be misconstrued down the road.' "
And for the defense attorney who has had killers as clients, there is another question. Horowitz himself repeated, and answered it, on the "Abrams Report."
"Could I stand next to someone who I knew committed a murder like this?" he said. "I don't know. Right now, I guess I'd have to say it would be a pretty hard thing to do."
Horowitz told Court TV's Nancy Grace in an interview that he's satisfied with the way the media have treated him.
"The only thing I want from the media," he said, "is what they've done, which is they put on her picture. They put on our friends who talk about how beautiful she was. And that's it. OK. They've done what I need them to do. What they do with me doesn't matter."