A lone attacker who killed one officer and hurt another in a stabbing went to a police station just hours earlier and expressed hatred for them, Belgian authorities said.
The suspect made “unhinged remarks” during a talk with officers, Brussels prosecutor Sarah Durant said.
Because he voluntarily asked for psychological help, he was not arrested and instead sent to a hospital – which he left soon after.
Immediately after the attack – which is being investigated as possibly terror-related – the suspect was shot by another police officer.
He is currently being treated in hospital.
Authorities came under immediate criticism for not arresting the suspect when he made his initial threats to the police early on Thursday.
However, Ms Durant said procedures stipulate that he had to be taken by police to a hospital since he had asked for psychological help himself.
“Police remained there until hospital staff took over,” she said.
A few hours later, when police checked on his whereabouts, “it appears that the person had left the hospital”.
Eric Van der Sypt of the federal prosecutor’s office said investigators are looking into whether the attack in Brussels happened in a “terrorist context”.
According to Belgian media, the suspect shouted “Allahu Akbar”, the Arabic phrase for “God is great”, during the attack, which happened at about 7.15pm on Thursday.
Belgian authorities kept the overall terrorist threat level at two out of a possible four, meaning the risk of an attack is still “medium”.
Belgian Prime Minister Alexander De Croo posted a message of condolence to the family and friends of the dead officer.
“Our police officers risk their lives every day to ensure the safety of our citizens,” he said.
“Today’s tragedy demonstrates this once again.”
Interior minister Annelies Verlinden said she was in contact with the Brussels mayor, police chief and security services to co-ordinate a response to the attack.
“Such violence against our people is unacceptable,” she said.