The suspect in the murder of police sergeant Matt Ratana has been named as 23-year-old Louis De Zoysa, according to reports.
New-Zealand-born Sgt Ratana, 54, died in hospital after being shot at the Croydon custody centre, south London, as a handcuffed suspect was taken into custody on Friday morning.
A source told the BBC that De Zoysa, from Norbury, south-west London, is the suspect in Sgt Ratana's murder.
De Zoysa is thought to have shot himself and is critically ill in hospital.
Another man was arrested in Norwich, Norfolk, on suspicion of supplying a firearm. He is currently in custody.
Police have not yet been able to speak to De Zoysa as part of the murder investigation. He had initially been arrested for an alleged drugs offence and possession of ammunition.
The shots were fired as officers prepared to search the suspect, who was still handcuffed, with a metal detector, according to watchdog the Independent Office for Police Conduct.
The second arrest came as Sgt Ratana was remembered by friends and teammates at East Grinstead Rugby Club, where he was head coach.
The club's vice chairman Matt Marriott said they had to arrange two separate minute's silences because the "interest has been pretty enormous" with "people coming from all over the country".
PC Sarah D'Silva, who plays for the club's women's team as well as working at Croydon Police Station, said it felt "extremely poignant" to join the minute's silence.
She wore her police uniform to pay her respects to Sgt Ratana, who she described as "an absolutely fantastic character, full of life, with the biggest smile you've ever seen".
Metropolitan Police Commissioner Dame Cressida Dick, speaking at the National Police Memorial in central London, said she "hadn't been surprised at all" by the number of tributes paid to him.
"Matt was an extraordinary person... he had a wonderful personality and he was very good at his job," she said, adding that he was a "proud Kiwi".
Sgt Ratana was shot in the chest at around 2.15am on Friday morning.