Image Bruno Gulotta. He had been holding his son’s hand when he was struck by the van. He worked for an online publication in Italy. Credit... Tom's Hardware

By Friday, investigators were starting to piece together a broad plot that involved a network of terrorists. Five suspects were killed in a shootout with the police in the seaside town of Cambrils, south of Barcelona, and a woman injured in Cambrils later died of her injuries. At the same time, the grim task of identifying the dead gathered speed as relatives struggled to learn whether loved ones were among those injured, missing or killed.

The State Department declined to identify the American killed in the attack “out of respect for the family’s privacy in their time of grief.”

It is unclear whether the department was referring to Jared Tucker, a 42-year-old American construction worker from Lafayette, Calif., who has been missing since the attack, according to his wife, Heidi Nunes-Tucker. Ms. Nunes-Tucker, a 40-year-old teacher, said she and her husband had been on a vacation to celebrate their first wedding anniversary, and were drinking on a patio moments before the van began to plow through tourists on Las Ramblas.

Ms. Nunes-Tucker said that her husband had gone to the bathroom, and that she was pushed inside a souvenir kiosk and then quickly ushered away from the scene by Spanish police. She has not seen her husband since.

“I can’t understand why this would be happening at all,” she said, speaking from a Spanish courthouse where she had been summoned by the police. She was accompanied by an official from the American Consulate, and was waiting to learn whether her husband had been killed.

“If that is, in fact, my husband — he was just full of life and the most magnetic person,” she said. “Everyone wanted to be around him, and so for him to not be here just doesn’t make any sense whatsoever to me. My heart is just broken. He’s the love of my life.”

There were reports that a 7-year-old child with dual nationalities may be missing, but these could not be independently confirmed.

Image Marta Scomazzon, who worked in a press office, and Luca Russo, an engineer. They lived in Italy.

Prime Minister Theresa May of Britain said her government was investigating reports of a missing child who was a British dual citizen.

Among those killed was Elke Vanbockrijck, a 44-year-old mother from Tongeren, Belgium, who was with her husband and two young sons in Barcelona, according to Belgian news reports. The mayor of Tongeren, Patrick Dewael, confirmed that a female victim was from Tongeren, but he did not identify her. News media reports identified her either as Elke V. or by her full name.

During the attack, Ms. Vanbockrijck’s husband pushed their two small boys aside, although one was injured, according to reports. His wife died at the scene, said the Dutch newspaper Het Belang Van Limburg.

Ms. Vanbockrijck had worked as an office clerk at the Belgian postal company. Her husband is a soldier in the Belgian Army, the newspaper reported.

Didier Reynders, the Belgian minister of foreign affairs, posted on Twitter that two Belgians were also hospitalized, with “one in a critical state.”

The nature of the international threat was emphasized by Mr. Dewael, a lawmaker who is a presiding member of a parliamentary commission investigating last year’s deadly terrorist attack in Brussels, where the main airport and a subway station were bombed.

“This reminds of the attacks that happened here,” he said, adding: “Absolute security does not exist. An attack like yesterday in Barcelona cannot be prevented.”

Image Elke Vanbockrijck, of Belgium, was with her husband and young sons in Barcelona. Credit... City of Tongeren, via Associated Press

In Italy, Prime Minister Paolo Gentiloni identified two victims: Luca Russo, 25, and Bruno Gulotta, 35. He said that all Italians mourned them with their families.

Mr. Gulotta, a marketing and sales manager and father of two, died in front of his partner and two young children, who narrowly escaped being hit by the van, according to news agency reports.

The Italian media reported that Mr. Gulotta had been holding the hand of Alessandro, 5, when he and his partner, Martina Sacchi, who had their baby, Aria, strapped to her chest in a carrier, heard a noise. Mr. Gulotta instinctively tried to shield his son, but was hit just as Ms. Sacchi pulled the boy away.

Mr. Gulotta’s death was confirmed by a manager on the website of the online publication where he worked, Tom’s Hardware Italia.

“Bruno put his soul into his work while managing to divide his life between his job and his family,” Andrea Ferrario, the chief operating officer and editor in chief of Tom’s Hardware, said in a telephone interview. “Everything he did, all the passion he put in at work, he did to improve the condition of his family,” he said.

Mr. Ferrario said he had just turned on the news when he sat down for dinner on Thursday and first heard of the attack. “I thought to myself, Bruno is in Barcelona. Never in a million years would I have thought that he could have been involved,” he said.

Not long after, he heard through a colleague that Mr. Gulotta had been killed.

The other Italian victim, Mr. Russo, was in Barcelona for a vacation with his girlfriend, identified as Marta Scomazzon, who was also struck by the van and remained in the hospital.

Image Ian Moore Wilson

Reached at his home, Mirco Scomazzon said, “My daughter is fine,” but he declined to speak further. Italian news media reported that Ms. Scomazzonn had fractured a foot and an elbow.

The two lived in Bassano del Grappa in northeastern Italy. Mr. Russo had an engineering degree from the University of Padua. Riccardo Poletto, the mayor of Bassano del Grappa, who teaches religion at the high school where Mr. Russo went to school, remembered him as a good student.

The mayor said Ms. Scomazzon had worked as an intern in the City Hall press office last year and she also sang in a local choir. Mr. Poletto ordered all flags in the city to fly at half-staff until the funeral.

“We’re also planning a moment of commemoration for Luca, to demonstrate solidarity with the family,” he said.

The Spanish authorities said a 60-year-old man, Francesco Lopez Rodriguez, of Granada, died in the attack. His wife was severely injured.

In Canada, Prime Minister Justin Trudeau announced that one Canadian citizen had been killed in the Barcelona attack and that four others had been injured.

On Friday night, the Canadian was identified as Ian Moore Wilson, by his daughter Fiona Wilson, a Vancouver police sergeant. In a brief statement posted on the Vancouver Police Department’s website, Sergeant Wilson “thanked the people who assisted my dad in his final moments,” and those who gave “urgent medical attention and aftercare” to her mother, Valerie. The couple had had been married for 53 years.