Seriti says he found no reason to not give Pistorius the prescribed minimum sentence.
Oscar Pistorius's prison sentence for the 2013 murder of his girlfriend Reeva Steenkamp has been more than doubled by a South African appeals court.
South Africa's Supreme Court of Appeals increases prison sentence for former Olympic sprinter to 13 years and five months for the murder of girlfriend Reeva Steenkamp.
Judge Masipa initially found Pistorius guilty of culpable homicide and in October 2014 sentenced him to five years imprisonment, but re-sentenced him to six years after the state appealed and the SCA in December 2015 found him guilty of murder.
Pistorius claimed he thought she was an intruder.
According to the BBC, he was handed the lighter sentence of six years because of mitigating factors such as his "rehabilitation and remorse", which "outweighed aggravating factors such as his failure to fire a warning shot".
Spokesperson for Ms Steenkamp's parents Tania Koen told Associated Press that they were emotional as they watched the judgment on TV.
It remains unclear if Pistorius's legal team will appeal this most recent decision to the highest courts possible.
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The NPA said the six-year sentenced was shockingly low and if they had allowed it to stand, it would have set a bad precedent.
Pistorius, 30, shot and killed Reeva Steenkamp in the early hours of Valentine's Day in 2013 when he fired four times through the door of his bedroom toilet.
Pistorius' brother, Carl, wrote on Twitter: "Shattered". He served only 10 months of the five-year sentence in prison before being released and put under house arrest.
"Heartbroken. Gutted." A spokesman for the Pistorius family didn't answer calls from the AP.
The Organisation Justice for Victims tweeted that 13 years was "still no justice". He is taken to Pretoria prison.
Steenkamp's family said at the time they were "satisfied" with the five-year sentence, even if it wasn't what they were hoping for.
"He is a person well-trained in the use of firearms and was holding his weapon at the ready in order to shoot", the court's decision read.