Announcing the latest arrests, Halil Ibrahim Dasoz, the Malatya governor, gave no details, but said they were all of the same age group - young men aged 19 and 20.
Turkish newspapers said all the suspects detained at the crime scene were found carrying a letter that read: "We did it for our country. They are trying to take our country away, take our religion away."
Nationalists had previously held a demonstration outside the publishing house, accusing it of proselytising, the private Dogan news agency reported.
The Zirve publishing house has been the site of previous protests by nationalists accusing it of proselytising in the 99 per cent Muslim but secular country, the news agency said.
Zirve's general manager told CNN-Turk television that his employees had recently been threatened.
"We know that they have been receiving some threats," Hamza Ozant said, but could not say who made the threats.
Police have yet to make a statement on the motives for the murders, but the press on Thursday were united in the belief that nationalist and religious zeal were the likely root cause.
The manner in which the victims were bound suggested that the attack could have been the work of a local Islamic group, said commentators.
CNN-Turk said police were investigating the possible involvement of Turkish Hezbollah - a Kurdish Islamic organisation that aims to form a Muslim state in Turkey's Kurdish-dominated southeast.
Turkish Hezbollah - which has been known to "hog-tie" its victims while torturing them - takes its name from the better-known Lebanon-based Hezbollah, but has no formal links to it. Turkish authorities recently said they were witnessing an increase in the group's activities.
Of Turkey's 70 million people today, only about 65,000 are Armenian Orthodox Christians, 20,000 are Roman Catholic and 3,500 Protestant - mostly converts from Islam. Another 2,000 are Greek Orthodox Christians.