The brothers, who say they are innocent, are accused of having formed an organised criminal network in early 2021 in Romania, as well as in the US and Britain
On Tuesday, authorities in California charged a suspected serial killer with the murder of three men in a string of shootings that have shocked the local community, officials said.
Truck driver Wesley Brownlee, 43, was arrested on Saturday in the city of Stockton, around 130km east of San Francisco, on suspicion of killing six men and wounding a woman over the past 18 months.
Police said that Brownlee had been potentially seeking another victim when he was arrested.
District attorney Tori Verber Salazar said on Tuesday that she expected to file more charges in the other murders that Brownlee was suspected of carrying out.
"We're waiting for additional evidence to be processed through that will most likely ... allow us the opportunity to add those additional charges," said Salazar.
"We're only four days into this," she added.
Five men were killed in one area of Stockton between July 8 and September 27, and the murders bore a number of similarities.
Another man was killed in Oakland, about 80km away, last year.
One woman also survived a shooting believed to have been carried out by Brownlee.
The victims ranged in age from 21 to 54.
Investigators, including the FBI, are working to piece together a possible motive, and to identify any other potentially-related homicide cases.
Several of the victims were homeless, and the murders — committed late at night or at dawn — are all linked by ballistics, police said.
Salazar said that Brownlee, who moved to Stockton this summer, has a criminal record, including drug-related felony offences.
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