The prosecutor's office in Sophia said the five provided "logistical" support in carrying out the 13 November attack.
ADVERTISEMENTFive suspects have been indicted in Bulgaria in connection with a deadly bomb blast in Istanbul on 13 November, which Turkish authorities blamed on Kurdish PKK fighters and their allies in Syria.
"Five people have been charged with helping one of the alleged perpetrators of the attack to flee," said Siyka Mileva, spokeswoman for the Sofia prosecutor general.
Their role was "logistical", she said, adding that the prosecutor's office had requested detention for four of them.
According to local TV stations, three were from Moldova and one from an unspecified Arab country.
Six people were killed and 81 injured in an explosion on a pedestrian thoroughfare in the heart of Istanbul on Sunday afternoon.
The attack has not been claimed at this stage.
So far, 17 people have been detained in Turkey, including a 23-year-old woman accused of planting the bomb on a bench in Istiklal Avenue.
Alham Albashir was arrested at an apartment in an Istanbul suburb, and is believed to have entered Turkey illegally from north-eastern Syria, according to the authorities.
She admitted to the crime while in custody, they said.
According to the official Anadolu news agency, citing the police report, she said she first came into contact with the Kurdistan Workers' Party (PKK) in 2017 through her ex-boyfriend and allegedly kept a link with the organisation after they broke up.
The PKK and the YPG (People's Protection Units), a Kurdish militia active in Syria and accused by Turkey of being affiliated with the PKK, have denied any involvement in the attack.