LONDON — British investigators said Monday that at least one of the three assailants in the weekend terrorist attack in London was previously known to security officials, and opposition politicians challenged Prime Minister Theresa May’s record on security matters.
After a brief display of national unity and with just days to go before the parliamentary election, rivals zeroed in on May’s record as home secretary.
While May has promised to overhaul the country’s counterterrorism strategy after the third terrorist incident in three months, critics pointed out that she was in charge of security for six years before replacing David Cameron as prime minister.
During that time, she oversaw a reduction in police forces, including armed officers.
“I am so sick of Theresa May blaming others for terror when the system she presided over has obviously failed so lamentably,” Steve Hilton, once a close adviser to Cameron, wrote on Twitter. May, he added in a separate tweet, “should be resigning, not seeking reelection.”
Jeremy Corbyn, leader of the opposition Labor Party, when asked by a reporter if he would support calls for May’s resignation over the falling number of police officers, replied, “Indeed, I would.”
Stung by the criticism just days before the election Thursday that will decide her political future, May responded at a news conference. “We have protected counterterrorism policing budgets,” she said. “We have also provided funding for an increase in the number of armed police officers.”
She cited comments from Cressida Dick, head of the Metropolitan Police in London, that the city’s force is well resourced and has powerful counterterrorism abilities.
The prime minister also came to the defense of Sadiq Khan, mayor of London, who was mocked by President Trump over his response to the attack.
The Metropolitan Police released the names and photographs Monday of two of the assailants they shot to death, saying one of them was known to the intelligence services.
In a statement, they said they identified one of the suspects was Khuram Shazad Butt, 27, a British citizen who was born in Pakistan and known to MI5, the agency that deals with internal security.
The other man was identified as Rachid Redouane, 30, who had claimed to be Moroccan and Libyan but who had also used the name Rachid Elkhdar and a different date of birth.
“Khuram Shazad Butt was known to the police and MI5,” the statement said. “However, there was no intelligence to suggest that this attack was being planned, and the investigation had been prioritized accordingly. The other named man, Rachid Redouane, was not known.”
Work was still underway to identify the third man who was killed during the attack on Borough Market.
The statement added that investigators are trying to “understand more about them, their connections and whether they were assisted or supported by anyone else.” The Islamic State has claimed responsibility for the plot, although it has previously claimed attacks by people who were inspired by the militant group but not directly connected with it.
Butt was a known radical Islamist who was filmed in a British television documentary unfurling a black flag resembling the one used by the Islamic State, the Associated Press reported. He raised the suspicion of a neighbor after allegedly trying to lure local youngsters to join his jihadist campaign.
Neighbors from Barking, in East London, identified Butt from the film’s footage of ‘‘The Jihadis Next Door,’’ which aired on British television last year.
One of them, Erica Gasparri, had contact police about 18 months ago, the AP reported. The 42-year-old mother of three was working at a local school when she noticed Butt meeting with local children and trying to draw them into his radicalism.
‘‘It was wrong what he was doing,’’ Gasparri said. ‘‘He kept talking about the Islamic State.’’
Early Monday, police raided two homes in East London, but there were no details on what those searches revealed.
On Monday evening, thousands of people gathered at Potters Field, across from the Tower Bridge and the medieval Tower of London, to pay tribute to the victims.
The gathering provided a kaleidoscope of London’s diversity, with Buddist monks in saffron robes, Christian clerics in purple cassocks, and Muslims in black T-shirts bearing the words ‘‘I am a Muslim: Ask me anything.’’
Speaking to those gathered, Khan decried the attackers, saying: ‘‘You will not win. We will defeat the terrorists.’’
The main political parties suspended campaigning Sunday as a sign of respect for the seven people killed and the scores wounded in the attack, but as campaigning resumed Monday, so did the pressure on May.
Although there has been widespread praise for the professionalism and courage of the armed officers who shot and killed the assailants within eight minutes of being called Saturday night, the country’s broader antiterrorism strategy was questioned.
Opposition politicians focused their fire on May, who gave a short speech outside her office on Downing Street on Sunday saying that “enough is enough,” promising to shake up antiterrorism and deradicalization policies, and calling for new efforts to curb the dissemination of extremist materials on the Internet.
Some of May’s political opponents regarded her comments as political and, as a result, in breach of the agreement to suspend campaigning.