BRUSSELS — Belgian commandos and bomb disposal units swept through a district at the heart of the Brussels attack probe Friday, underscoring the widening security fears as prosecutors acknowledged that they missed a chance to press a key terrorist suspect for intelligence in the days ahead of the twin-site suicide bombings.

Even as a trio of bombers were racing to strike, fearing that authorities were closing in on them after the March 18 detention of one of their key allies, Salah Abdeslam, investigators failed to concentrate on interrogating Abdeslam about his knowledge of future attack plots, prosecutors said Friday.

A police officer wears a bomb suit in Brussels during a day of police raids and investigations. A tram passing through the area was stopped and evacuated and police cordoned off a wide perimeter of streets. The Associated Press

Instead, they ran through a recitation of their understanding of his involvement in the November strikes in Paris that killed 130 people, prosecutors said.

The conversations took place over two hours last Saturday, and authorities did not interrogate Abdeslam again until Tuesday, after the attacks, when he refused to speak further, prosecutors said.

The failure to push Abdeslam for concrete intelligence – even though key associates were known to be on the loose – adds to an emerging picture of failures by intelligence agencies, police forces and criminal investigators to take advantage of opportunities to avert the attacks Tuesday, the worst single day of violence in Belgium since World War II.

Friday’s raids were carried out as Secretary of State John Kerry and European leaders looked to sharpen strategies against the Islamic State. Other police operations in France and Germany displayed the expanding crackdowns that increasingly connect the last two terrorist blows in Europe: November’s bloodshed across Paris and Tuesday’s bombings in Brussels that killed at least 31 people.

Among those arrested in the latest roundups was a French suspect who officials believe was directing a plot for an impending attack in France. The investigation touched off a series of related police raids in Belgium.

Meanwhile, the list of the Brussels victims became clearer.

At least two Americans were killed, a U.S. official said Friday, but their names remained undisclosed.

Also among the dead from the airport bombings: a Dutch brother and sister who lived in the United States. They were Alexander Pinczowski, 29, and Sascha Pinczowski, 26, said a representative for their family, James Cain.

Cain, the father of Alexander Pinczowski’s fiancée and a former U.S. ambassador to Denmark, said the siblings had hoped to become U.S. citizens. Britain, the Netherlands, China and France also confirmed at least one citizen each among the fatalities.

“We will not rest until we have eliminated your nihilistic beliefs and cowardice from the face of the Earth,” Kerry said, directing his remarks at Islamic State-connected attackers who have struck around the world.

Kerry met with Belgian Prime Minister Charles Michel before joining a Europe-wide security meeting to examine ways to counter militant reach into the continent.

Officials have raised alarms about potential threats from citizens returning after fighting with the Islamic State and other groups.

Even as Kerry and the others gathered to assess the problems, police fanned out just miles away.

One large raid appeared to concentrate on the Brussels district of Schaerbeek, the same area where some of the attackers stayed and where police later found an apartment filled with bombmaking material.

Police detained one person, the Belgian federal prosecutor said, and witnesses reported hearing explosions, apparently from bomb squad robots.

Belgian TV aired amateur footage of the detention that appeared to show a man who had been shot in the leg being dragged away from a tram stop by black-clad counterterrorism police while a bomb-disposal robot waited nearby. Belgian prosecutors said the man was arrested in connection with a French raid a day earlier.

In Germany, authorities held a man who was deported from Turkey to Europe in July alongside Brussels suicide attacker Ibrahim el-Bakraoui over suspicions of trying to fight in Syria. A German official said it was not immediately clear whether the man detained Thursday had direct ties to Bakraoui.

Both Ibrahim and Khalid el-Bakraoui, the brothers who blew themselves up Tuesday, were on a U.S. terrorism watchlist ahead of the attack, according to a U.S. official. It was not clear whether they had been on the U.S. “no-fly” list.

Earlier, a new suspect in the attacks was identified as Naim al-Hamed, a 27-year-old Syrian man born in Hama. He was described as “very dangerous, suspected of being armed,” according to a police notice detailed in Belgian media.

Hamed was suspected of involvement in both attacks, but it was not immediately clear whether he was the elusive third attacker who authorities believe dropped a suitcase with explosives at the Brussels airport and then vanished.

Belgium’s federal prosecutor said Friday that a man detained in a raid the previous night in the Paris suburb of Argenteuil is believed to have connections to Abdelhamid Abaaoud, the deceased ringleader of November’s Paris attacks that left 130 dead.

The 34-year-old French citizen, Reda Kriket, had been convicted in a Belgian court in July for participating in the activities of a terrorist group, the prosecutor said.

Three more raids in Brussels were conducted in connection with Kriket’s arrest, according to the prosecutor.

French Interior Minister Bernard Cazeneuve said that Kriket was “at the advanced stage” of plotting an attack on the country.

But Cazeneuve said there was no apparent link to Tuesday’s attacks in Brussels, which served as a hub for militants planning the Paris massacres.

At the same time, police pressed ahead with a manhunt for a suspected accomplice who is believed to have fled Tuesday’s attack at Brussels Airport.

The French newspaper Le Monde and the Belgian broadcaster RTBF reported that video monitors captured images of another possible accomplice, who is believed to have slipped away on the Brussels subway. The report could not be immediately confirmed.

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