Security failures may have played a role in the stabbing death of a policeman during an extremist attack on Parliament last year in London, Britain's chief coroner said Wednesday.

Coroner Mark Lucraft said "shortcomings in the security system" meant armed officers weren't guarding an entrance to Parliament when an extremist stabbed policeman Keith Palmer to death on March 22, 2017. Palmer was one of five people killed by 52-year-old Khalid Masood in the attack on Westminster Bridge and Parliament. Masood was then shot dead by police.

Speaking at an inquest, Lucraft said armed officers were not aware they were supposed to remain "in close proximity" to the gates outside the Parliament building. He said if they had been stationed there "they may have been able to prevent" Palmer from suffering fatal injuries.

The officer's widow, Michelle Palmer, said her husband "was left at a vulnerable location with no protection to die."

"How could Keith have been left alone, unarmed, guarding an open gate at one of the most iconic buildings in the world and one of the country's top terrorist targets?" she asked.

She charged that "nobody wants to take responsibility" for leaving one of their officers unprotected.

Speaking outside the courthouse after the coroner's findings, Neil Basu, chief of counter-terrorism at the Metropolitan Police, said the force "unreservedly accepted" the coroner's conclusions.

"Even the possibility that the Met (police) lost the chance to prevent the murder of such a brave and courageous officer is unacceptable," he said. "For the loss of that possibility, to protect him from Khalid Masood, we are deeply sorry."

Basu said major improvements to the security system at Parliament have been implemented.

The Parliament attack was the first in a series of extremist assaults on British soil in 2017, during which dozens were killed. It was followed by the May bombing of a concert hall in Manchester that killed 22 victims; a June attack on London Bridge and Borough Market that killed eight victims and an attack in the same month on a mosque in Finsbury Park in London that left one person dead.

Britain's official threat level is set at "severe," indicating that officials believe an extremist attack is highly likely.