SkyNews is reporting they've taken the second suspect to an "undisclosed hospital":
Metropolitan Police officials said the 33-year-old suspect was arrested after handing himself in at a south London police station.
He was later transferred to an undisclosed hospital.
A 21-year-old man also questioned in connection with the deaths was released earlier without charge.
The Guardian is saying this:
A Metropolitan police spokesman told the Guardian the man was taken to hospital after handing himself in at a south London police station this morning.
"It is a precautionary measure," the spokesman added. "Whenever anyone is taken into custody, they are asked a series of questions and, if they have any visible injuries, they are dealt with."
The spokesman said the suspect - the second man to be arrested in connection with the double murder inquiry - had yet to be interviewed about the killings.
So it looks like they're not going to be saying they released the first guy because the second guy was clearly the perpetrator....
I bet they also want a psych consult on this second guy, without weakening any potential case by saying that's what they want. Note the exact phrasing of the police spokesman; he didn't actually say this particular suspect had any 'visible injuries'..