GUATEMALA CITY, March 20 (Reuters) - Four suspected drug traffickers were arrested in Guatemala on Tuesday and accused of masterminding the murder of three Salvadoran politicians, a twist in a scandal that has put pressure on the government.

The violent, election-year drama has also claimed the lives of four Guatemalan detectives who were arrested for the murders and found days later with their throats slit in a prison cell.

Last month, the bodies of three lawmakers from El Salvador and their driver were found bullet-ridden and charred on a Guatemalan back road.

Authorities say drug gangs with links to Guatemala and El Salvador were behind the killings, which have shed light on illegal armed groups within Guatemala's security forces and possible links between high-level officials and narcotics traffickers.

Opposition politicians say Interior Minister Carlos Vielmann has allowed death squads to operate within the police force. Vielmann has offered to resign over the scandal.

Prosecutors said the woman and three men arrested on Tuesday in the Jutiapa region that borders El Salvador were caught on camera at a gas station buying fuel used to burn the Salvadoran lawmakers' car.

"We have information that they masterminded the killing and were looking for drugs or money," Vielmann told a news conference as the suspects were brought to court.

The first to be arrested had been the four policemen, including a senior organized crime investigator.

After they were killed in jail, witnesses and human rights investigators said armed men had entered the prison through the front door.

Authorities blamed gang members inside the prison for the deaths, saying they used smuggled weapons and collaborated with guards to kill the policemen.

A fifth policemen turned himself in soon after his colleagues were killed.

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