Harsh accusations have flown between Turkey and Syria, signaling a sharp escalation of already high tensions between the two former allies. But Prime Minister Recep Tayyip Erdogan suggested that Turkey would not be drawn militarily in retaliation.
He insisted Turkey would "maintain our extreme cool-headedness in the face of efforts and provocations to drag us into the bloody quagmire."
"Those who target Turkey will be held to account sooner or later," he said. "Great states retaliate more powerfully, but when the time is right... We are taking our steps in a coolheaded manner."
Turkish Foreign Minister Ahmet Davutoglu told reporters in Berlin those detained were linked to a Marxist terrorist group.
Sabah, a Turkish newspaper close to the government, reported Sunday that authorities suspect the leader of a former Marxist group, Mirhac Ural, now believed to be based in Syria, may have revived his group and ordered the attack.
The group, Acilciler, was one of many Marxist groups active in Turkey through the 1970s and 1980s, and was long-rumored to have been formed by the Syrian intelligence agency. Many of its militants allegedly included ethnic Arab Turks belonging to a sect close to Syria's Alawites.
"Some believe that now that relations (with Turkey) have deteriorated again, Syria may have reactivated the group to cause turmoil in Turkey," said Nihat Ali Ozcan, a terrorism expert at the Ankara-based Economic Policy Research Foundation of Turkey.
Guler said a ringleader was among those detained, and more arrests were expected.
"We have determined that some of them were involved in the planning, in the exploration and in the hiding of the vehicles," he said.
Saturday's twin bombings 15 minutes apart damaged some 735 businesses and 120 apartments, leaving smoking hulks of buildings and charred cars. It also wounded dozens of people, including 50 who remained hospitalized Sunday.
Syria and Turkey became adversaries early on during the uprising against Syrian President Bashar Assad that erupted in March 2011. Since then, Turkey has firmly sided with the Syrian opposition, hosting its leaders along with rebel commanders and providing refuge to hundreds of thousands of Syrian refugees.
Deputy Prime Minister Besir Atalay said the aim of the attack was to stoke tensions between Turks and Syrian refugees. The town is home to members of Turkey's Arab Alevi community, an offshoot of Shiite Islam, while many of the refugees who have fled Syria are Sunni.
On Sunday, hundreds of people marched in the city of Antakya, near Reyhanli, protesting the government for its Syria policies and support for the rebels which some believe has exacerbated the conflict in Syria. Turks in Hatay, the southern province where the town is located, complain that the rebels roam freely, disrupting calm in Turkey's border regions.
Witnesses said they saw Turks attacking Syrian registered cars in Reyhanli soon after Saturday's attack and some Syrians avoided going out in the streets. Erdogan asked citizens in Reyhanli to remain calm and not "fall for the provocations."
"The prime minister brought this on to us," said a business owner, who asked to be identified only by his first name, Mehmet. "We have no peace anymore. The Syrians are coming in and out, and we don't know if they are bringing in explosives, taking out arms."
Authorities had so far identified 35 of the dead, three of them as Syrians. Families began burying their loved ones in funerals on Sunday.
Earlier in Damascus, Syrian Information Minister Omran al-Zoubi rejected Turkey's charges, saying that "Syria didn't and will never undertake such acts because our values don't allow us to do this."
He accused Turkey of destabilizing the border areas between the two countries by supporting the rebels, who the regime has labeled terrorists.
"They turned houses of civilian Turks, their farms, their property into a center and passageway for terrorist groups from all over the world," Al-Zoubi said. "They facilitated and still are the passage of weapons and explosives and money and murders to Syria."
Al-Zoubi also branded Erdogan a "killer and a butcher," adding that the Turkish leader "has no right to build his glory on the blood of the Turkish and Syrian people."
Tensions had earlier flared between the Syrian regime and Turkey after shells fired from Syria landed on the Turkish side, killing five Turks, and prompting Germany, the Netherlands and the U.S. to send two batteries of Patriot air defense missiles each to protect their NATO ally.
Davutoglu said his country would hold those responsible for the bombings but had no immediate plans to involve its NATO allies.
The attacks come just a little over a week after Israel escalated its role in the Syria conflict by striking suspected shipments of advanced Iranian weapons in Syria.
Erdogan is flying to the U.S. for talks with President Barack Obama next week. In the wake of the car bombs, both men could come under greater pressure to take action.
"It comes down to an existential struggle," said Salman Shaikh, director of the Brookings Doha center. "Those who oppose Assad really have to show that they mean it now."
The U.S. has provided humanitarian aid to the Syrian opposition, but has been reluctant to provide military aid, in part because al-Qaida-linked militants are becoming increasingly influential in the armed opposition.
Last week, Erdogan alleged that Syria has been using chemical weapons, delivering them on at least 200 missiles, though he provided no evidence. Syria has denied using chemical weapons.
Obama has portrayed the use of chemicals by the regime as a "red line" that would have harsh consequences, but has said he needs more time to investigate allegations.
In another potentially destabilizing element, Israel signaled last week that it will keep striking at shipments of advanced Iranian weapons that might be bound for Hezbollah. Syria has traditionally be a conduit for Iranian weapons to Hezbollah.
Earlier this month, Israel struck twice at what Israeli officials said were shipments of advanced Iranian missiles near Damascus. In response, Hezbollah chief Hassan Nasrallah said this week that Syria is expected to deliver "game-changing" weapons to his militia. If more than empty rhetoric, this would likely provoke more Israeli strikes.
Associated Press writers Albert Aji in Damascus, Karin Laub and Zeina Karam in Beirut and Geir Moulson in Berlin contributed.