The new witness, scheduled to give evidence on Friday, has information about an attack in which two men had their legs shot at, reported the TT news agency.
Mangs was sentenced to life in prison in 2012 for two murders and five attempted murders.
In the run up to the trial, which put an end to what Malmö residents called a period of terror as an unknown gunman targeted mostly foreign-born victims in the southern city, the police attempted to tie Mangs to several more attacks but inconclusive forensic evidence stood in their way.
Mangs, who has been diagnosed with Asperger’s Syndrome, now wants to have the convictions overturned.
In November 2012, he was also convicted of vandalism, violent assault and perverting the course of justice.
However, the Malmö District Court was not unanimous in its decision, with one lay judge stating Mangs should only be convicted of one murder.
He was acquitted of several charges, including a third murder, another five attempted murders, and one count of conspiracy to commit murder.
All in all, he was charged with 12 attempted murders in which he allegedly fired numerous shots with his Glock 19 pistol at homes, businesses and cars as well as out in the open, seriously injuring a number of people and coming close to killing many others.
Mangs “has ruthlessly ruined many people’s lives. He is completely indifferent (to their suffering). It is clear that this has been a lifestyle for Peter Mangs,” chief prosecutor Solveig Wollstad told the court at the time.
From the start, Mangs has protested his innocence.