The Belgian-led investigation believes a check-in counter for an American carrier was one of the targets in the March 22, 2016 attacks.
The Belgian-led investigation believes a check-in counter for an American carrier was one of the targets in the March 22, 2016 attacks, the sources said on condition of anonymity.
They also suspect that travellers to Israel may have been in the crosshairs, and that airport security camera footage shows one bomber apparently pursuing Hasidic Jews seconds before one of the blasts.
Islamic State (IS) bombers Najim Laachraoui and Ibrahim El Bakraoui killed 16 people at Zaventem airport. Around an hour later Bakraoui's brother Khalid attacked a metro station near EU headquarters, killing another 16.
One source close to the investigation told AFP, which contacted investigators in several countries, that one of the airport bombers "attacked the Delta Airlines check-in".
"We know they wanted to target Americans," said the source, who asked not to be named. "It's clear they had quite specific targets."
Asked if these targets included the check-in counter for a flight to Israel, he replied: "We know they were obsessed with the Israelis too."
The possibility that they targeted Russian travellers was an "option" that had to be clarified, he said.
There were casualties from at least 40 nationalities in the Brussels attacks, but investigators and sources believe the Zaventem bombers had specific targets.
"Even early on (in the investigation) there were indications that they targeted US, Russian and Israeli check-in counters," a US law enforcement source told AFP on condition of anonymity.
"That understanding has held up with later investigations, including with Abrini's alleged confession," the source said.
Mohamed Abrini is the so-called "man in the hat" who fled the airport without detonating his suitcase bomb after his accomplices set off theirs. He was captured a month later.
Four Americans were killed at the airport and several injured, while two Israelis aged 23 and 28 were treated in Jerusalem after the attacks.
Israeli media identified the pair as members of the Belz Hassid ultra-Orthodox religious sect, who had been scheduled to fly from Brussels airport to Ben Gurion.
Sources close to the investigation added that camera footage never released to the public showed that Laachraoui had been standing among some 60 high school students before deciding to pursue two Orthodox Jews.
"The attacker seemed to rush towards two Orthodox Jews," one of the sources said. "He really, clearly wanted to kill a Jew."
A US government source said separately that Lachraoui was targeting a Hasidic Jew. Hasidic men are easily recognisable because they wear dark suits over white shirts, have long beards, hats and curly sidelocks.
Investigators "are very confident they (bombers) were targeting US, Russia and Israel," the US government source said.
Israeli security measures have dramatically increased since the attacks, an airport source added.
A Belgian press report said investigators had found an electronic message from Khalid El Bakraoui saying flights departed every Tuesday from Brussels to the United States, Russia and Israel, but investigators would not confirm that.
However the attack did occur around 8:00 am on a Tuesday, shortly before scheduled flights to all three countries by United, American, Delta, El Al, Brussels Airlines and Russia's Aeroflot, according to airport and other sources.
An airport source who asked not to be named said the bomb that did not explode was left near the United and El Al counters, which were facing each other.
Abrini appeared in unreleased airport CCTV footage as lagging behind the other bombers before hiding behind a pillar covering his ears, two independent sources said.
"Every indication was that Abrini changed his mind," the US law enforcement source said.
Investigators have said the bombers were part of the same Brussels-based cell that orchestrated the November 2015 Paris attacks.
They believe they were spurred into action in Brussels by the March 18 arrest of key Paris attacks suspect Salah Abdeslam.
"Completely cornered and hunted, they clearly improvised," a source close to the Belgian investigation said, adding that one of the bombers left a message on an abandoned computer apologising for not having launched a new attack on France.
Belgium had also suffered an attack on the Jewish Museum in Brussels in May 2014 which left four people dead. The alleged attacker, Mehdi Nemmouche, has been linked to Abdelhamid Abaaoud, the ringleader of the Paris attacks.
Israeli foreign ministry spokesman Emmanuel Nahshon said "Israel does not comment publicly on security-related issues" when asked about the March 2016 attacks. The FBI, which is taking part in the investigation, did not respond to a request for comment.
Russia's investigation committee, which probes major crimes in Russia, said it is "not investigating the Brussels attacks."