Police in Sweden are therefore concerned that the situation could escalate following what appears to have been a liquidation in Copenhagen last week, in which the two victims' Swedish-registered car was riddled with shots from multiple weapons.
As a result, police presence has been stepped up in three Stockholm neighbourhoods in a bid to prevent further violence.
“Such an act almost always has consequences. That is something we are trying to prevent,” Stephan Kiernan, leader of Swedish police special unit Team 2022, told Svenska Dagbladet.
The police force will increase presence in the Rinkeby, Tensta and Kista areas in Stockholm.
One of the victims of last week's shooting, a 23-year-old, has been reported by several media to have a leading role in a gang known as Shottaz, which is connected to Rinkeby. The individual was manager for a local rapper who is currently in prison for weapons offences, according to Svenska Dagbladet.
Danish police appeared to have made a breakthrough in the case on Saturday, when a 21-year-old Swedish-speaking man was arrested in Aarhus before appearing in court at Glostrup near Copenhagen.
The nature of the charges against the man have not been made public.
Newspaper Ekstra Bladet published images showing the dramatic nature of the Aarhus arrest, showing a man climbing outside an apartment building two storeys above ground level.
That man belongs to another gang, Dödspatrullen, according to the Danish newspaper's report. The two Swedish crime groups have reportedly been in conflict since 2015, with seven people losing their lives as a result. Only one of those seven cases – the shooting of a man in a pizzeria in Rinkeby – has been solved by police.
Earlier this year, Svenska Dagbladet published an investigation in which it mapped out the structure of the two groups, in which 20 central figures have been convicted of 330 crimes.
A common feature of the gang members is their young age and serious nature of their crimes. The shooter in the pizzeria murder was just 16 years old.
All of the 20 individuals were known to social services in Stockholm, but efforts to steer them away from crime appear to have been ineffective.
“Some of them have little faith in the future,” a policeman close to the issue was quoted as saying in Svenska Dagbladet's report.
Copenhagen West Police superintendent Flemming Madsen declined on Sunday to comment further on the Danish investigation of the Herlev shooting.
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