Belgium raised the death toll from the Brussels attacks to 35 and charged three terror suspects Monday as police across Europe stepped up efforts to unravel an Islamic State network linked to the carnage.

Police released a new video of a third suspect in the March 22 Zaventem airport attack, the so-called "man in the hat" seen with two other suicide bombers, who escaped after his bomb failed to explode.

The U.S. State Department on Sunday confirmed the deaths of two more Americans in the Brussels suicide attacks, bringing the total number to four.

"We can confirm the deaths of two additional U.S. citizens in Brussels, and we express our deepest condolences to their loved ones," a State Department official told AFP.

The murder this week of a security guard at a Belgian nuclear research center is a purely criminal affair unrelated to any terrorist threat, prosecutors said Saturday.

The Charleroi prosecutor's office in charge of the case "formally denied" any connection between the murder of Didier Prospero, who was shot dead on Thursday at his home in Froidchapelle and a planned terror attack, the Belga news agency reported, citing the prosecutor.

Brussels airport will not reopen before Tuesday as it implements new security measures and repairs the departure hall wrecked by a suicide bomb attack earlier this week, it said Saturday.

"Passenger flights at Brussels Airport will not resume before Tuesday 29 March," Zaventem airport said in a statement.

Belgium's federal prosecutor confirmed Friday that Najim Laachraoui was one of the two suicide bombers who struck Brussels airport this week in a series of attacks which left 31 people dead.

The prosecutor also linked Laachraoui to November's Paris carnage in which 130 people died, saying his DNA was found on a suicide vest and a piece of cloth discovered at the Bataclan concert hall where 90 people were killed.

A Mormon missionary who survived the blasts at the Brussels airport says he's lucky to be alive after previous brushes with fate at the Boston Marathon bombings and the France terror attacks.

Mason Wells was standing in line at a Delta check-in counter when the first explosion went off just feet (meters) away, he told CNN on Friday.

Belgian police shot a suspect as part of a huge European terror crackdown that netted several arrests Friday as France's president said a jihadist network that targeted both Paris and Brussels was being "destroyed".

Grieving Belgians held prayers in the rain in a central Brussels square carpeted with flowers and tributes to the 31 dead and 300 wounded in Tuesday's carnage, but there was also growing anger at the government for letting a string of militants slip through the net.

Najim Laachraoui was a good student with an immaculate disciplinary record at a Brussels Catholic school, but grew up to become a Syria-hardened bomb-maker at the heart of the Brussels and Paris terror attacks.

The 24-year-old was one of two men who Tuesday blew themselves up at Brussels airport -- the day after police identified him as a suspected accomplice whose DNA was found on explosives used in the French capital in November.

A British computer programmer who had just messaged his family to say he was safe; Dutch siblings phoning a relative just as the bombs went off: harrowing stories continue to emerge of the 31 killed in the Brussels attacks.

Reflecting the cosmopolitan nature of Brussels, the symbolic capital of Europe, victims came from as far afield as Peru, China and the United States, as well as neighboring France and the Netherlands.

U.S. Secretary of State John Kerry on Friday declared "Je suis Bruxellois" -- "I am a citizen of Brussels" -- in support for the people of the Belgian capital, echoing their backing for the United States after the 9/11 terror attacks.

"Then, voices across Europe declared, 'Je suis Americain.' Now, we declare, 'Je suis Bruxellois' and 'Ik ben Brussel'," Kerry said in French and Dutch, the country's two main languages, after meeting Belgian Premier Charles Michel.