Belgian authorities had close contact with some of the men believed to be behind the bloody terrorist attacks in Paris last week, a pattern that raises questions about how the suspects could slip through the fingers of law enforcement officials.

Over the past year, Belgian security forces tapped at least one bomber's telephone and briefly detained and interviewed at least two other suspects - one for his travels to Syria and the other for his radical views, according to law enforcement officials here.

State prosecutors also had been pursuing Abdelhamid Abaaoud, an alleged ringleader of the plot, for dragging his 13-year-old brother with him to join Islamic State militants in Syria.

On Wednesday, police and soldiers raided an apartment in a Paris suburb in search of Abaaoud, and at least two of its occupants died. They included Abbaoud, two senior European officials said.

Although the attackers struck in Paris, several were living in or had traveled to a single neighborhood in Brussels.

The high level of awareness about the men - and the missed opportunities for police and intelligence agencies to stop them before the attacks in Paris - shows how difficult it is for hard-pressed security services to keep track of hundreds or thousands of people and pick out which ones might be organizing violent acts.

Not only were police suspicious of the men tied to the Paris attacks, but Belgian researchers and even journalists also were tracking their posts on social media.

Belgian officials said that three men linked to the Paris attacks appeared on a list of 800 Belgians with suspected ties to terrorist groups, a list maintained by the Belgian Coordination Unit for Threat Assessment, a government advisory group. France has a similar list, with 1,200 names.

Yet some experts point out that Belgian security services are handicapped by a shortage of people to follow suspected extremists, a dearth of Arabic speakers, a history of lax gun laws and layers of overlapping government authority.

Guy Van Vlierden, a Belgian journalist who says he is tracking about 100 suspected Belgian extremists on social media, sympathizes with the overwhelmed police and intelligence services.

"We always see in hindsight that people were known or followed," he said. "Such things happen. In hindsight it is easy to say, but with the numbers that have to be followed, it's impossible to watch them all."

Still, the roll call of Paris attackers highlights how tantalizing close police came to identifying them.

Belgian authorities began to take an interest in Bilal Hadfi, one of the suicide bombers, shortly after his secondary school teachers became alarmed at his sudden swerve toward radical Islam.

Sara Stacino, a former teacher at a Dutch-speaking high school in the Molenbeek neighborhood, told VRT television that Hadfi was "a nice student, motivated and interested in politics, more than the others."

But his views became more extreme. "He stopped listening to music, and he believed that women should be veiled if they didn't want to be raped," Stacino said.

After the January attacks in Paris on the satirical magazine Charlie Hebdo and a kosher supermarket that left 17 dead, Hadfi expressed support in class for the massacre.

Stacino alerted the school administrators - in writing. "But we all took a rather cautious stand," she said.

Soon after, Belgian authorities began to track his movements, according to the Justice Ministry. Hadfi traveled to Syria in the spring. He was posting on Twitter from Syria in July under an alias. He referred to the Western coalition fighting the Islamic State in Syria and Iraq as "infidels" and wrote "they should no longer feel safe, not even in their dreams."

His activity on social media caught the attention of the journalists a few months ago. "I knew of him, but couldn't tell what he would do," Van Vlierden said.

"We knew Hadfi had traveled to Syria and had come back," said Seighild Lacoere, a Justice Ministry spokeswoman. For two weeks, the security services tapped the phone line of the house where he lived in Molenbeek, a neighborhood in Brussels populated by Muslim immigrants from North Africa.

"But when he wasn't found to be home, they had to stop the tapping, according to the legal requirements," she said.

In February, after Turkish authorities stopped Brahim Abdeslam, 31, a cafe and bar manager from the Molenbeek neighborhood, on his way to Syria and sent him home, the Belgian federal police took him in for questioning, according to Eric Van Der Sypt, spokesman for Belgium's federal prosecutor.

About the same time, police also interrogated Brahim's brother Salah, 26, who authorities believed had been recently radicalized, according to Van Der Sypt.

But then the Belgian police let them go. There was, Van Der Sypt said, no evidence that they would engage in acts of terrorism, so "that was it."

At least for the moment. Brahim and Salah Abdeslam were allegedly part of the attacks Friday in Paris, with Brahim blowing himself up on Rue Voltaire and Salah becoming the subject of a massive manhunt. A total of 129 people were killed, not including the assailants.

Belgian authorities have scored some successes. In January, counterterrorism police conducted a nationwide sweep, killing two suspected Islamist militants, arresting more than a dozen and disrupting an alleged plot to target police.

In the January raid against a safe house in the town of Verviers, about 75 miles east of Brussels, counterterrorism agents found police uniforms, forged IDs, explosives and automatic weapons.

Several of the suspects whom the police were searching for had traveled to Syria to fight with Islamist militants. One of them was Abaaoud, the man who would later be named as the inspiration behind last week's Paris attacks. Abaaoud was not apprehended.

"It's not as though the Belgians are incompetent," said François Heisbourg, a security expert and chairman of the International Institute for Strategic Studies.

He noted that Belgian security officials alerted the French and American governments several years ago of a plot to detonate a car bomb outside the U.S. Embassy in Paris.

Heisbourg said Belgian security services are hampered by the way minorities are not integrated into Belgian society. That means that those plotting terrorism can live in "safe havens" within crowded cities.

In largely Moroccan Molenbeek, very few police officers speak Arabic, and many of them come from Dutch-speaking cities and aren't as comfortable speaking French. For much of this year, the police station in Molenbeek set up barriers to keep people a safe distance from the entrance.

Parents in Molenbeek, who may hear their children express interest in going to Syria, often do not want to turn to the police with tips, analysts said. "They don't trust the police," said Montasser AlDe'emeh, a researcher on Islamist extremism who also runs a center to discourage young Muslim Belgians from going to Syria.

And he said that those young people who do go to Syria, often return and try to stay inconspicuous.

"They know that ISIS war will continue for years," AlDe'emeh said. "They can wait two years before executing an operation."