Istanbul (CNN) Turkish officials have strong evidence that ISIS leadership was involved in the planning of the Istanbul airport terrorist attack, a senior government source told CNN on Thursday.

Officials believe the men -- identified by state media as being from Russia, Uzbekistan and Kyrgyzstan -- entered Turkey about a month ago from the ISIS stronghold of Raqqa in Syria, bringing with them the suicide vests and bombs used in the attack, the source said.

The terrorists rented an apartment in the Fatih district of Istanbul, where one of the attackers left behind his passport, the Turkish government source said.

The attack was "extremely well planned with ISIS leadership involved," the source said.

Turkish police visited the Fatih area and showed neighbors video and photographs of three men they say are believed to have carried out the attack, residents said.

Police showed residents of Fatih this image of three suspects.

Among the pictures was a screen grab from surveillance video at the airport that shows three men wearing thick jackets and carrying bags, the residents said. One man, the owner of the Bekir Yar Emlak real estate agency, said he told police that the picture showed the men who had lived in his apartment. He said he was shocked to learn they could be the suspects.

Mustafa Elsan, the owner of a garage next to the apartment building, also said he was shown images of the alleged attackers. Elsan said he would see people smoking by the window of that apartment on the first floor of the building, but the curtains were mostly closed.

A female neighbor in the same building, who didn't want to disclose her name, said there was some smell of chemicals a couple of days ago from the apartment, prompting other neighbors to ask her whether there was a gas leak.

Photos: Istanbul airport attacked People flee the scene of a terror attack at Istanbul's Ataturk airport on Tuesday, June 28. Three terrorists armed with bombs and guns attacked the main international terminal, opening fire and eventually detonating their devices. Hide Caption 1 of 22 Photos: Istanbul airport attacked Investigators remove a body after the attack. Hide Caption 2 of 22 Photos: Istanbul airport attacked Police investigators work inside the airport. Hide Caption 3 of 22 Photos: Istanbul airport attacked Turkish special forces secure an area of the airport after the attack. Hide Caption 4 of 22 Photos: Istanbul airport attacked A wounded girl is taken to a hospital in Istanbul. Hide Caption 5 of 22 Photos: Istanbul airport attacked A Turkish police officer directs a passenger at the airport. Hide Caption 6 of 22 Photos: Istanbul airport attacked Travelers embrace outside the airport. Hide Caption 7 of 22 Photos: Istanbul airport attacked One of the bombs was located just outside the international terminal on the pavement, Turkish Justice Minister Bekir Bozdag told CNN. Another was at the security gate at the entrance to the airport. Hide Caption 8 of 22 Photos: Istanbul airport attacked Children and their relatives embrace after reuniting outside the airport. Hide Caption 9 of 22 Photos: Istanbul airport attacked A police officer sets up a security perimeter. Hide Caption 10 of 22 Photos: Istanbul airport attacked People stand outside the airport after the attack. Hide Caption 11 of 22 Photos: Istanbul airport attacked A wounded woman talks on the phone following the attack. Hide Caption 12 of 22 Photos: Istanbul airport attacked Passengers cry as they leave the airport. Hide Caption 13 of 22 Photos: Istanbul airport attacked People on their phones wait with their luggage outside the airport. Hide Caption 14 of 22 Photos: Istanbul airport attacked Workers clear glass debris on the day after the attack. Hide Caption 15 of 22 Photos: Istanbul airport attacked Bullet holes are seen at the airport on Wednesday, June 29. Hide Caption 16 of 22 Photos: Istanbul airport attacked Hide Caption 17 of 22 Photos: Istanbul airport attacked A worker cleans blood from the upper walls of the international departure terminal. Hide Caption 18 of 22 Photos: Istanbul airport attacked A police officer stands guard as a man walks at the airport a day after the attack. Hide Caption 19 of 22 Photos: Istanbul airport attacked A woman cries in Istanbul on June 29. Hide Caption 20 of 22 Photos: Istanbul airport attacked Security personnel scan passengers and employees at a checkpoint on June 29. Hide Caption 21 of 22 Photos: Istanbul airport attacked A worker repairs the airport's damaged ceiling on June 29. Hide Caption 22 of 22

Death toll climbs

The death toll rose to 44 when a 3-year-old Palestinian boy hurt in the attack died, according to Palestinian officials. The boy's mother died Wednesday, officials said. And Turkish state news agency Anadolu reported a Turkish man died at a hospital.

"One of the toughest battalions in ISIS is called the Uzbek battalion," he said. "These were the guys who were essentially on the front lines guarding Falluja, the city they just lost in Iraq."

"Ask anybody inside ISIS or who's fought ISIS. People from the former Soviet Union tend to be the most battle-hardened and willing to die," he said.

CNN national security analyst Peter Bergen said the revelation of the Istanbul attackers' nationalities will serve to "open people's eyes to the fact that there's a very substantial Russian, former Soviet Union, presence within ISIS, both in terms of the foot soldiers and the leaders."

Estimates of Russian fighters involved with ISIS range from 2,000 to 7,000, he said.

Kyrgyzstan's foreign ministry disputed reports that one of the attackers was from that country.

The ministry said Turkish officials told its representatives that "the identities of the suicide bombers are still being examined."

Suspects detained in attack probe

Also Thursday, authorities detained 22 people in connection with the attack, according to a Turkish official.

JUST WATCHED Anatomy of an explosion Replay More Videos ... MUST WATCH Anatomy of an explosion 01:11

Thirteen were taken into custody in Istanbul and nine in the coastal city of Izmir, the official said. Three of those detained were foreign nationals, state media reported.

The terrorists stormed the airport Tuesday nigh t, opening fire and detonating explosives -- two of them at the international terminal building, and the third in a parking lot, according to officials.

History of airport attacks

Preliminary findings suggest all three attackers at Ataturk airport opened fire and then detonated explosives strapped to their bodies, similar to the mass shootings and suicide bombings at Paris' Bataclan concert hall in November. ISIS claimed responsibility for that massacre , which left 89 people dead. Similar attacks took place elsewhere in Paris the same night, killing an additional 41 people.

The tactic -- to enter shooting, and then detonate explosives -- is called "inghimasi," and it's being used more and more frequently by terrorists.

"The 'inghimasi,' their (modus operandi) on the ground in Syria and Iraq, is to shoot up checkpoints and then they actually -- some of these guys actually run up to the enemy and hug them before detonating the bomb to take them out with themselves. So in a sense, the ultimate Kamikaze warrior," Weiss said.

JUST WATCHED Istanbul airport attacked despite heavy security Replay More Videos ... MUST WATCH Istanbul airport attacked despite heavy security 03:01

ISIS also has a history of airport attacks. It claimed responsibility for dual suicide bombings at the main airport in Brussels in March. At least 10 people died in those blasts.

The CIA director said the terror attack mirrors similar ones by ISIS.

"I think what they do is they carry out these attacks to gain the benefits from it in terms of sending a signal to our Turkish partners ... and at the same time not wanting to potentially maybe alienate some of those individuals inside of Turkey that they may still be trying to gain the support of," John Brennan said.

Victims from all over the world

Those killed at the Istanbul airport had come from all over the world, but most of them were Turkish, including 10 airport employees, TAV Airports CEO Sani Sener said.

Saudi Arabia's foreign ministry said six Saudis were killed and dozens more wounded.

Other fatalities included two Iraqis, one Tunisian, one Chinese, one Iranian, one Ukrainian, one Jordanian and one person from Uzbekistan, a Turkish official said. Three of the foreigners had dual Turkish citizenship.

One U.S. citizen suffered "minor injuries," Homeland Security Secretary Jeh Johnson said Thursday.

Turkey's vulnerability

The Islamic State has struck in Turkey before, but has rarely taken credit for those bombings.

Experts say Turkey is especially vulnerable because various terrorists operate there.

ISIS has a reason to detest Turkey. The country is helping the U.S.-led coalition attack ISIS targets in neighboring Iraq and Syria. Turkey allows coalition planes to fly raids from its territory.

Adding to the list of enemies, Turkey resumed hostilities with the PKK -- Kurdish militant separatists -- last year after a ceasefire broke down.

Turkey has spent much of this year reeling from terror attacks as it weathers bombing campaigns by both ISIS and Kurdish militants.

The airport attack marked the eighth suicide bombing in Turkey this year. At least 140 people have been killed. The violence has also rattled Turkey's tourism industry, a key sector of the national economy. About 39.4 million people visit each year.