BRUSSELS — As police hunted for accomplices in the terorist bombings in Brussels, the Belgian justice and interior ministers acknowledged Thursday that authorities had erred by not acting on Turkey’s request that they take custody of a Belgian civilian arrested for suspected terrorist activity.

The man arrested last year was one of the Islamic State suicide bombers in Tuesday’s deadly attacks in Brussels, authorities said.

The acknowledgments by Justice Minister Koen Geens and Interior Minister Jan Jambon were the first high-level Belgian admissions of blunder in the aftermath of the bombings that killed 31 people, plus the three suicide bombers, and wounded about 270.

About a dozen Americans were injured in the bombings, according to the State Department, which said it was persuing reports from American families who said they had relatives missing in Belgium.

State Department spokesman Mark Toner said Thursday that Belgium had not released the nationalities of those who died.

An intense search continued for one of the Brussels airport attackers who was recorded on a airport surveillance video and had fled.

Prosecutors declined to comment on reports from Belgian state broadcaster RTBF and France’s Le Monde and BFM television that a fifth attacker may also be at large: a man seen on surveillance cameras in the Brussels metro carrying a large bag alongside one of the suicide bombers. It is not clear whether that man was killed in the attack or is a fugitive.

With at least one attacker at large and an unknown number of accomplices, police detained six people in raids around the Belgian capital Thursday night, the Associated Press reported.

The raids targeted central Brussels, Jette, and the Schaerbeek neighborhood, where police earlier had found a huge stash of explosives and bomb-making material in an apartment used by the Brussels attackers.

In a Paris suburb, a man suspected of plotting an imminent attack was also detained Thursday, but the interior minister reported no apparent link with the Brussels airport and subway bombings or the Nov. 13 attacks on Paris.

Authorities lowered Belgium’s terror-threat level by one notch, although they said another attack is ‘‘likely and possible.’’ Belgium had been on its highest alert ever since the Brussels airport and subway attacks.

The attacks have exposed missteps by European security officials and police, just four months after the Islamic State’s assault on targets in Paris.

“What’s essential in the story is that with the passing on of the information from Turkey and with the passing on of the information within Belgium, we have been slower than one could have expected under those circumstances,’’ Geens said on Flemish television in Belgium. “So, the information was passed on, but we have not been diligent, or probably not diligent enough.’’

Jambon told the newspaper Le Soir that there had been “two types of mistakes, at the level of the Justice Ministry and at the level of the liaison officer in Turkey, which involves the Interior and Justice ministries.’’

Both ministers offered their resignations Thursday, an implied acknowledgment of responsibility for perhaps having failed to avert the attacks. Prime Minister Charles Michel rejected the resignation offers.

But questions proliferated about what law enforcement authorities did — and failed to do— to thwart the bombers, who appeared to be part of the same Islamic State network that carried out the Paris attacks in November.

The most glaring lapse, which Turkey’s president first raised publicly Wednesday, appeared to be Belgian officials’ inaction on a Turkish request in June that Belgium take custody of Ibrahim El Bakraoui, who had been arrested in Turkey as a suspected terrorist for trying to enter Syria.

Bakraoui, who had spent time in a Belgian prison for attempted robbery and shooting at a police officer, was one of two brothers identified as being among the three suicide bombers in Brussels.

When Belgium did not act on the notification, Turkish officials said, they deported Bakraoui to the Netherlands at his request.

The other suicide bombers were identified as Bakraoui’s brother, Khalid, and Najim Laachraoui.

It remains unclear why Belgian officials did not order Bakraoui reincarcerated, since his mere presence in Turkey appeared to have violated terms of his release from prison.

The Justice Department of the Netherlands confirmed Thursday that Bakraoui had been sent to Amsterdam from Istanbul on July 14, the same day it had been informed by Turkey’s Foreign Ministry in a “very urgent’’ notification message, but that the message did not explain the nature of the urgency and raised no alarms about him.

The connections between the deadly attacks are pointing investigators and terrorism experts toward the conclusion that there is a large network of trained attackers that leads back to Syria but is rooted in Europe as well.

“There seems to be more and more evidence that there are links between French commandos who had a role in Paris and Belgians who targeted the airport and the Maelbeek metro station,’’ said Didier Leroy, a researcher on jihadi networks at the Belgian Royal Military Academy and Brussels University.

“Definitely there are other attacks to be feared and other individuals will emerge,’’ he said.

Both the Paris and Brussels assailants appeared to have shared a bomb-maker, Laachraoui, 24, a Belgian of Moroccan descent, the second suicide bomber at the Brussels Airport. The Belgian prosecutor’s office has not confirmed his death, saying “it is too early to say,’’ but European intelligence officials said he was dead.

The Brussels attacks came less than a week after police in Belgium captured Salah Abdeslam, believed to be the sole surviving participant in the Paris attacks in November that killed 130 people.

Investigators have since linked him to some of the Brussels bombers. His lawyer, Sven Mary, told reporters at a court hearing in Brussels Thursday that Abdeslam wanted to be extradited to France, reversing his earlier position.

Abdeslam was Europe’s most-wanted man until he was arrested Friday in a raid in Molenbeek, the Brussels neighborhood where he grew up.

Asked why Abdeslam had changed his mind, Mary said that his client understood “the case here is just a small piece,’’ and that he wanted to “explain himself in France.’’ Abdeslam has not spoken to investigators since the bombings in Brussels.

Asked what his client had said about the attacks, Mary replied, “He didn’t say, because he didn’t know it.’’ Asked whether Abdeslam had reacted to the attacks, Mary responded, “He had no reaction.’’

The federal prosecutor in Belgium said Thursday that a day earlier, investigators had searched the homes of the Bakraoui brothers. He also said that an arrest warrant had been issued for Khalid El Bakraoui on Dec. 11 by the judge investigating the Paris attacks.

“I wouldn’t want him to clam up,’’ Mary said. “His clamming up would make us face other Zaventems and other Bataclans.’’ He was referring to the Belgian airport that was bombed and to the Paris concert hall where 90 people were killed by gunmen on Nov. 13.