BRUSSELS — Counterterrorism officials widened a sweep targeting suspected Islamic State operatives to several European countries on Friday, reporting newly uncovered links between the Brussels and Paris massacres, at least five arrests, and the foiling of what France described as an advanced plan for another attack.
The actions reflected both new momentum from information uncovered since the Brussels bombings on Tuesday and deep worries about missed opportunities to thwart the attacks. European officials, particularly in Belgium, have come under strong criticism for lapses that might have enabled the Brussels plotters to succeed.
President François Hollande of France, who has declared a state of war with the Islamic State, praised the police work carried out in recent days but said that “we know that there are other networks,” affiliated with the extremist organization and lurking in Europe.
“Even if the one that carried out the attacks in Paris and in Brussels is in the process of being wiped out — with a certain number of its members arrested — there is always a threat weighing upon us,” Hollande said in Paris.
Hollande’s warnings were reinforced by a newly released Islamic State propaganda video featuring what were described as two fighters from Belgium, apparently speaking from Iraqi territory seized by the organization and celebrating the Brussels attacks. “This is just the beginning of your nightmare,” one fighter proclaims, according to a translation by the SITE Intelligence Group, which tracks extremist organizations.
European investigators already had established numerous links between suspects in the Paris and Brussels attacks, but the Belgian prosecutor’s office confirmed the most direct connection on Friday, saying DNA matches showed that one of the bombers who blew himself up at the Brussels airport had been a bomb maker who helped produce two suicide vests used in the November Paris attacks, which killed 130 people.
The man identified as the bomb maker — Najim Laachraoui, 24, a Belgian citizen — has been described by the Belgian prosecutor’s office as an accomplice of Salah Abdeslam, 26, who was captured in Belgium on March 18 and charged with terrorist murder. Abdeslam is thought to be the sole surviving direct participant in the Paris attacks, and his arrest appeared to have accelerated the plot that culminated in the attack on Brussels, which killed 31 people.
US officials confirmed Friday that at least two Americans were killed in the Brussels attacks and said there could be more. The names of the US victims have not been released. But a Dutch brother and sister who lived in New York — Alexander Pinczowski, 29, and Sascha Pinczowski, 26 — were among those killed, according to a spokesman for their families.
A couple from Kentucky, Stephanie and Justin Shults, are still missing, their relatives said.
Secretary of State John Kerry made a somber visit to the Belgian capital Friday and promised that the United States will step up efforts to block future attacks and to destroy the Islamic State.
‘‘The United States is praying and grieving with you for the loved ones of those cruelly taken from us, including Americans, and for the many who were injured in these despicable attacks,’’ Kerry said.
Laachraoui, the bomb maker, traveled to Syria in February 2013, the Belgian prosecutor’s office said. Last September, while using a false identity card, he and Abdeslam were stopped at the Hungarian-Austrian border, but not detained, according to the prosecutor’s office.
He rented a house in Auvelais, Belgium, that was used by the attackers, and traces of his DNA were found in an apartment in the Schaerbeek section of Brussels that he used as a bomb-making lab, the prosecutor’s office said.
On Monday — three days after Abdeslam was captured in Molenbeek, the Brussels neighborhood where he grew up — the authorities asked for the public’s help in finding Laachraoui.
A day later he blew himself up at Brussels Airport, along with another suicide bomber, Ibrahim El Bakraoui, 29.
News agencies widely reported Laachraoui’s death, but officials awaited DNA results before confirming it
On Friday, the federal prosecutor in Brussels not only confirmed the death, but also disclosed that Laachraoui’s DNA had been found on suicide vests detonated in Paris.
The investigation also spread to Germany, where officials reported a suspected link between El Bakraoui and a 28-year-old German in Dusseldorf who had ties to Islamist extremists and was about to serve a prison term for robbery. He was arrested Thursday night to prevent him from fleeing to Syria, according to the prosecutor’s office for North Rhine-Westphalia state.
Turkey deported the German man and El Bakraoui to the Netherlands last year on the same flight, German news agencies and security officials said.
German officials also disclosed on Friday that they had arrested a 28-year-old Moroccan, who had a criminal record in Italy, during a routine identity check at a train station in Giessen, near Frankfurt.
The man appeared to have circumstantial links to the Paris and Brussels assailants, the officials said.
According to the weekly news magazine Der Spiegel, a text message on a cellphone carried by the Moroccan man contained the name of Khalid El Bakraoui, 27 — Ibrahim’s younger brother and the man identified as the suicide bomber at the Brussels subway station on Tuesday.
On Thursday, the French police arrested Reda Kriket, who, according to court records, is a jihadi who raised money for a network of militants in 2012 and ’13 and traveled to Syria in late 2014.
Kriket was well known to the security services in both France and Belgium, and he was named in a 2015 court proceeding along with Abdelhamid Abaaoud, the on-the-ground chief planner of the Paris attacks.
In July, Kriket was convicted in absentia in Belgium of terrorist activities and possession of stolen goods.
After Kriket’s arrest, the French interior minister, Bernard Cazeneuve, said Kriket had been involved in the “advanced stages” of a new terrorist plot. It was unclear whether that plot was directly connected to the attacks in Paris or Brussels.
In Brussels on Friday, the police arrested three men for questioning in connection with Kriket’s arrest.
One was shot and arrested in the Schaerbeek neighborhood, in an operation that traumatized the area, which was still unnerved from the bombings.