ISTANBUL -- As the death toll from the Istanbul airport attack rose Thursday to 44, a Turkish official confirmed to CBS News the three suicide bombers who carried it out were from Russia, Uzbekistan and Kyrgyzstan, and police raided neighborhoods for suspects linked to the Islamic State of Iraq and Syria, or ISIS.
A U.S. law enforcement source told CBS News that authorities believe that the attackers were trained and fought on the battlefield in Syria and possibly in Iraq because of their tactics, CBS News senior investigative producer Pat Milton reports.
Turkish authorities have said all information suggested the Tuesday night attack on Ataturk Airport, one of the world's busiest, was the work of ISIS, which boasted this week of having cells in Turkey, among other countries.
The police raided 16 locations in three neighborhoods on both the Asian and European sides of Istanbul, rounding up 13 people suspected of having links to ISIS.
There was no immediate claim of responsibility by the militant group, which has used Turkey as a crossing point to establish itself in neighboring Syria and Iraq. ISIS has repeatedly threatened Turkey in its propaganda publications, and the NATO member has blamed ISIS for several major bombings in the past year in both Ankara and Istanbul.
President Recep Tayyip Erdogan, appearing at the opening of a suspension bridge in northwestern Turkey with Prime Minister Binali Yildirim, made a vague reference to "forces" that don't want the country to succeed and are using terrorism.
He referred to the separatist Kurdistan Workers' Party, or PKK, the Syrian Kurdish militia YPG and ISIS.
"They have been let loose against us by the forces who hold their leashes," Erdogan said. "The bombs that explode in our country today will tomorrow explode in the hands of those who sent them."
Kyrgyzstan's Foreign Ministry denied that an attacker came from that country, saying its representatives had talked to Turkish officials who said the identities were still to be determined.
Asked about the possible involvement of a Russian in the attacks, Kremlin spokesman Dmitry Peskov said he had no information on the issue.
There was no comment from Uzbekistan.
Russian President Vladimir Putin has said that between 5,000 and 7,000 people from Russia and other nations of the former Soviet Union have joined ISIS in Syria and Iraq.
Many Muslims from Russia's southern region of Chechnya have settled in Turkey since the time of the Chechen separatist wars, and Moscow has repeatedly accused Turkey of failing to cooperate in tracking down suspected terrorists.
People from Chechnya and other provinces in Russia's volatile North Caucasus region have had a visible presence among ISIS fighters. Tarkhan Batirashvili, who took the nom de guerre Omar al Shishani, or Omar the Chechen, an ethnic Chechen from the former Soviet republic of Georgia, rose to the rank of a senior ISIS commander before he died of wounds suffered in a U.S. airstrike in Syria earlier this year. Al-Shishani served as a magnet for jihadi fighters from the former Soviet Union.
The U.S. law enforcement source told CBS News that an estimated 1,000 Chechens have joined ISIS.
Turkish state media said the death toll in the attack rose to 44 after a 25-year-old airport worker succumbed to his wounds.
Interior Minister Efkan Ala said the dead included 19 foreigners. Dozens from the 230 people initially reported wounded are still hospitalized.
Two memorial services for victims were held at the airport, one of them honoring taxi drivers slain in the attack. Five funerals were held elsewhere, including for four members of the Amiri family.
Abdulmumin Amiri escaped death because he went to look for a taxi while his relatives watched their luggage. "At that time, the bomb went off," he told The Associated Press. "I was about four or five meters away."
Nilsu Ozmeric wept over the coffin of her fiance, Jusuf Haznedaroglu, a 32-year-old airport worker who was fatally wounded while waiting for a bus to go home.
"The wedding was next week," sobbed his mother, Cervinye Haznedaroglu, as visitors offered condolences.
In Paris, Deputy Mayor Bruno Julliard said the Eiffel Tower would be illuminated in the red and white colors of the Turkish flag to honor the victims in Istanbul and as "a reminder of the unbreakable support" of his city.
The U.N. Security Council condemned the attack, saying that any acts of terrorism are criminal and unjustifiable and are "one of the most serious threats to international peace and security."
Unconfirmed details of the attack flooded Turkish media. The private Dogan news agency said the Russian attacker had entered the country one month ago and left his passport in a house the men had rented in the Fatih neighborhood.
A source familiar with the investigation confirmed to CBS News that the three attackers rented an apartment in Fatih, a neighborhood relatively close to the airport, and were living there in the weeks or months before the attacks.
The Karar newspaper, quoting police sources, said the attackers were part of a seven-member cell that entered Turkey on May 25. The assailants raised suspicions of airport security on the day of the attack because they wore winter jackets on a summer day, media reported.
Turkish media praised police officer Yasin Duma as a hero. He was wounded in an exchange of gunfire with one of the attackers and reportedly saved many lives by shouting, "Bomb!" and alerting others to get away.
Turkey's interior minister said the explosives were a mix of RDX, TNT and PETN that were "manufactured." That combination is military-grade, raising the question of how the attackers obtained the bombs, said Jimmie Oxley, a chemist and explosives expert at the University of Rhode Island.
The Dogan news agency broadcast video of the police raids showing a special forces team carrying what appeared to be a steel shield to protect it as it entered a building.
In separate police operations, nine suspects believed to be linked to ISIS were also detained in the coastal city of Izmir. It was not clear if the suspects had any links to the airport attack.
The Izmir raids unfolded simultaneously in the Konak, Bucak, Karabaglar and Bornova neighborhoods, according to the Anadolu Agency. Police seized three hunting rifles and documents relating to ISIS.
The report said the suspects were in contact with ISIS militants in Syria and were engaged in "activities that were in line with the organization's aims and interests," including providing financial resources, recruits and logistical support.
On June 25, security forces killed two suspected ISIS militants trying to cross the border illegally after they ignored orders to stop, local media reported. One of the militants was wanted on suspicion he was planning a suicide attack in Ankara or the southern city of Adana, Anadolu said.