Ten people have been arrested in Turkey in connection with the murder of three Protestant Christians who had their throats slit on Wednesday.
The three victims were found bound hand-and-foot in the office
The victims were employees at a publishing house in the eastern town of Malatya that prints bibles.
All the suspects are 19 or 20 years old, officials said.
The killings were an apparent attack on the country's Christian minority. Christians make up less than 1% of Turkey's Muslim-majority population.
Police arrested five more people on Thursday over the killings at the Zirve publishing house, doubling the number of suspects in custody to 10, the governor of Malatya said.
The three victims - a German and two Turkish citizens who had converted to Christianity - were found with their hands and legs bound and their throats slit.
Five men detained at the scene on Wednesday have been charged. Unconfirmed reports in the local media say they all lived at a student hostel run by an Islamic foundation.
Flat mates there said they were quiet - believers, but not overtly devout. Some Turkish newspapers say the suspected killers told police they acted to protect Turkey and Islam.
Nationalists had protested at the publishing house in the past, accusing it of involvement in missionary activities, local media reported.
Prime Minister Recep Tayyip Erdogan vowed to bring those responsible to justice and described the attack as "savagery".
Turkish Foreign Minister Abdullah Gul said the killings were "an attack against Turkey's stability, peace and tradition of tolerance".
"There have been similar attacks in the past. We will certainly take stricter measures," Mr Gul said.
Many commentators are drawing links to the murder of a Catholic priest by a teenage gunman last year and the shooting of Armenian Christian journalist Hrant Dink in January.
The BBC's Sarah Rainsford in Istanbul says in each case the killers were young, apparently Islamist ultra-nationalists.
There is a rising wave of nationalist feeling in Turkey, our correspondent reports, with Christian minorities complaining of pressure and harassment.