London (CNN) The ringleader of the London terror attack last week tried to rent a 7.5-ton truck, British police have said, indicating the attackers were looking to carry out a larger-scale assault.

Three men rammed into pedestrians on London Bridge on the night of June 3 before going on a stabbing spree at bars and restaurants at the nearby Borough Market. Eight people were killed and the three attackers were shot dead by police.

London Metropolitan Police said the trio's ringleader, Khuram Butt , attempted to rent the large truck online on the morning of June 3 but did not provide payment details when prompted.

Butt rented a white Renault van in the evening instead, using a recently activated mobile phone.

Large trucks have been used in other ramming attacks in Europe and have caused higher numbers of casualties. Mohamed Lahouaiej Bouhlel killed 86 people in Nice, France, when he plowed through a large crowd celebrating the national Bastille Day last July. In Berlin, Anis Amri rammed into a Christmas market, killing 12.

Butt out on bail at time of attack

Met Police confirmed to CNN that Butt was out on bail at the time of the attacks, having been arrested in a low-level fraud offense in October 2016.

He wasn't charged and was about to be released from bail because of insufficient evidence, the police said.

Two British counter-terrorism sources told CNN that the arrest was part of a drive by British police and the MI5 intelligence agency to take known Islamist extremists "off the streets" by going after them for crimes unrelated to terrorism. That drive was initiated two years previously.

An official involved in the efforts said the arrests focused on members of the pro-ISIS group al Muhajiroun and likened it to "going after Al Capone for tax evasion."

The police and other counter-terrorism agencies have come under scrutiny after it emerged that Butt was under investigation when he carried out the attack.

Pink knives and Molotov cocktails

Police have released images of three pink ceramic kitchen knives found on or near the assailants. The Ernesto brand knives are 12 inches long, and were found with tape wrapped around the handles and leather straps attached to them to keep the weapons on the attackers' wrists.

Police are calling on anyone with information about the ceramic knives.

"We've been working round the clock to understand what these men did in the lead-up to the attack but we need to know more about these unusual knives. Where have they come from? Where might the attackers have bought them from?" the Met's counter-terrorism command chief Dean Haydon said, calling on the public to contact police if they had any information on the weapons.

Police also released photos of the van the attackers used to mow down people on the bridge.

Three attackers mowed down pedestrians on London Bridge on June 3.

Inside, they found 13 apparent Molotov cocktails made from wine bottles and a possible flammable liquid. There also were two blowtorches.

They found a number of office chairs and a suitcase, and believe that the attackers had told family they were using the van to move as a cover story.

Police said the three men -- Butt , Rachid Redouane and Youssef Zaghba -- had driven up and down London Bridge twice before carrying out the attack, in what appears to have been a dry run.

Police have conducted a series of raids in connection with the attacks and made 18 arrests. Five people remain in custody and the other 13 have been released.

In a raid of an east London apartment rented by Redouane, police found an English-language copy of the Koran opened at a page describing martyrdom and materials that may have been used to make the Molotov cocktails.

Police have arrested 19 people in connection with the attack. Six men are still in custody and one has been released on bail.