Three employees of a publishing house that distributes Bibles were killed in the latest attack apparently targeting Turkey's Christian minority.
The attack added to concerns in Europe about whether this predominantly Muslim country - which is bidding for EU membership - could protect its religious minorities. It also underlined concerns about rising Turkish nationalism and hostility toward non-Muslims.
The three victims - a German and two Turkish citizens - were found with their hands and legs bound and their throats slit at the Zirve publishing house in the central city of Malatya.
Police detained four youths, aged 19-20, and also suspect a fifth, who underwent surgery for head injuries sustained apparently in trying to escape by jumping from a window at Zirve, authorities said.
The five suspects had each had been carrying copies of a letter that read "We five are brothers. "We are going to our deaths. We may not return," according to the state-run Anatolia news agency.
Prime Minister Recep Tayyip Erdogan condemned the attack, and said investigators were looking into whether there were other suspects or possible links with terror groups.
"This is savagery," Erdogan said.
The German and one of the Turkish victims were found already dead, and the third victim died after being taken to the hospital, Malatya Governor Halil Ibrahim Dasoz said. The German man had been living in Malatya since 2003, he said. Anatolia identified him as 46-year-old Tilman Ekkehart Geske.
It was the latest in a string of attacks on Turkey's Christian community - which comprises less than 1 percent of the 70-million population.
In February 2006, a Turkish teenager shot a Catholic priest dead as he prayed in his church, and two other Catholic priests were attacked later that year. A November visit by Pope Benedict XVI was greeted by several nonviolent protests. Earlier this year, a suspected nationalist killed Armenian Christian editor Hrant Dink.
Authorities had vowed to deal with extremist attacks after Dink's murder, but Wednesday's assault showed the violence was not slowing down.
German Foreign Minister Frank-Walter Steinmeier condemned the attack "in the strongest terms," and said he expected Turkish authorities would "do everything to clear up this crime completely and bring those responsible to justice."
German Chancellor Angela Merkel's Christian Democrat Party - which opposes Turkey's bid to join the EU - said the attacks showed the country's shortcomings in protecting religious freedoms.
"After today's murders, the Turkish government must allow itself to be asked whether it is doing enough to protect religious minorities," the party's general secretary, Ronald Pofalla, said in a statement, calling religious freedom a "fundamental human right."
"The Turkish state is still far from the freedom of religion that marks Europe," Pofalla said.
Turkey began EU membership negotiations in October 2005 but talks have stalled over Turkey's refusal to formally recognize EU-member Cyprus. The country is under intense pressure to improve human rights and to expand religious freedoms and free speech as part of its membership bid.
A group of 150 lit candles and unfolded a banner that read "We are all Christians" in downtown Istanbul to protest at the attack and show solidarity with the Christian community. But there was far less public outcry than with Dink's murder, which was followed by widespread protests and condemnations. More than 100,000 people marched at Dink's funeral.
Malatya is known as a hotbed of nationalists, and is also the hometown of Mehmet Ali Agca, who shot Pope John Paul II in 1981.
The Zirve publishing house has been the site of previous protests by nationalists accusing it of proselytizing in this 99-percent Muslim but secular country, Dogan news agency reported. Zirve's general manager said his employees had recently been threatened.
Anatolia said the five suspects were students preparing for university entrance exam, and were friends who lived at the same student residence in Malatya.
The manner in which the victims were bound suggested the attack could have been the work of a local Islamic militant group, commentators said, and CNN-Turk television reported that police were investigating the possible involvement of Turkish Hezbollah - a Kurdish Islamic organization 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 Lebanon-based Hezbollah, but has no formal links to it. Turkish authorities recently said they were witnessing an increase in the group's activities.
"These are fanatics who continue to be present in Turkey and who at a moment's notice emerge with these acts of absurd violence," Monsignor Luigi Padovese, the Vatican representative in Turkey, was quoted as saying by Italian news agency ANSA.
Constantinople - modern-day Istanbul - was the Christian Byzantine capital for more than 1,000 years until it fell to Muslim forces in 1453 and became the seat of the Muslim Ottoman Empire.
Of Turkey's 70 million people today, only about 65,000 are Armenian Orthodox Christians, 20,000 are Roman Catholic and 3,500 are Protestant - mostly converts from Islam. Another 2,000 are Greek Orthodox Christians.