LONDON (Reuters) - British police have spent years preparing the defense plan they put into action in London on Wednesday but have repeatedly said it would be difficult to stop a “lone wolf” attacker armed with unsophisticated weapons such as a car and a knife.

Police tape blocks access to Parliament Square the morning after an attack by a man driving a car and weilding a knife left five people dead and dozens injured, in London, Britain, March 23, 2017. REUTERS/Neil Hall

Since the 9/11 attacks on the United States and the 2005 London bombings, the counter-terrorism police, the domestic spy agency MI5, the foreign intelligence and eavesdropping agencies have worked together more closely than ever before. (Details of London attack in graphic tmsnrt.rs/2o9CrW6)

Boundaries have been broken down, joint meetings and information sharing has become routine and they have funding for more staff. Police said in the last four years they have thwarted 13 terrorism plots similar to the mass killings carried out by Islamist militants in Paris and Brussels.

But such close working links do not guarantee all plots will be thwarted and one former anti-terrorism chief said it was down to luck that Wednesday’s attacker was stopped before doing more damage.

British-born Khalid Masood, 52, was named by police as the attacker who drove into pedestrians, killing at least two people, before stabbing to death an unarmed officer outside parliament. He was shot dead by police.

Prime Minister Theresa May said he had been investigated by the MI5 security service “some years ago” as a peripheral figure over concerns about violent extremism.

She appeared to dispel suggestions that he should have been more closely monitored by MI5. The police said Masood, who used a number of other aliases, had been previously convicted but not for any terrorism offences.

“The case is historical – he was not part of the current intelligence picture,” May said. “There was no prior intelligence of his intent – or of the plot.”

Police said his first conviction was in November 1983 for criminal damage and his last was in December 2003 for possession of a knife.

Security services in Belgium and France have faced criticism at home and abroad for intelligence failures and their response to attacks in Brussels, Paris and Nice.

Intelligence officers have repeatedly said they cannot monitor every suspect and that the conflict in Syria had led to a steep increase in Islamic State militant attempts to incite attacks on the UK.

Islamic State claimed responsibility for the attack in a statement issued by its Amaq news agency, but it was not clear whether the attacker was directly connected to the jihadist group.

MI5, which employs about 4,000 people, has been fully mobilized in support of the police in the wake of the attack.

But the response of the police to Wednesday’s attack and the security around parliament has already faced scrutiny, with questions about how the knife-wielding attacker was able to get into the perimeter of parliament and stab an unarmed officer.

The attack took place in London’s most heavily guarded area where armed police routinely patrol. The new headquarters of London’s Metropolitan Police is also barely a stone’s throwaway.

PURE LUCK

Kevin Hurley, the former head of counter-terrorism for the City of London police force, said it was “pure luck” the attacker was stopped and shot dead by a specialist VIP protection officer.

He said it was time for all British police to be armed. According to government figures, only about 5 percent of British police are authorized to carry guns.

“Here we are still with unarmed police officers at the front gate of Parliament Square directly on the high street,” Hurley told BBC Radio Scotland.

“So even now, in the middle of London, we are still complacent about the nature of the threat we face. If you don’t stop these people immediately as we’ve just seen they go on to spread misery mayhem and murder wherever they are. I do wish we would wake up and see what’s going on in the world.”

Bernard Hogan-Howe, who stepped down as London police chief in February, has previously said they were struggling to recruit the extra firearms officers.

NEW TACTICS, NEW THREATS

London police say they constantly update their tactics to adapt to new threats. Attacks requiring little planning or expertise have recently joined the threat list alongside sophisticated al Qaeda plots and the prospect of marauding militants.

Armed police officers would now “go forward” to take on militants, Craig Mackey, the current acting London Police chief, told reporters on Thursday.

Just three days earlier, armed police staged an exercise on the River Thames to simulate their response to the hijacking of a tourist boat.

“The work we’ve done over many years, practicing and exercising for scenarios like yesterday has helped,” Mackey said.

But added: “Nothing prepares people ... for what the reality of what occurred yesterday.”

Defence Secretary Michael Fallon agreed there was little the police could do when faced with something that required little planning or technical expertise.

“This kind of attack, this lone wolf attack, using things from daily life: a vehicle, a knife, are much more difficult to forestall,” Defence Secretary Michael Fallon told BBC TV.

“We’re also dealing with an enemy, a terrorist enemy, that is not making demands or taking people hostage, but simply wants to kill as many people as possible. This is a new element to international terrorism.”