Seventeen people have been injured after two cars crashed at a "car cruise" gathering and ploughed into spectators.
The vehicles collided on Monkswood Way, Stevenage, at about 21:45 BST on Thursday leaving two seriously injured and 15 more hurt.
One of the event's organisers described the "horrific" crash as a "nightmare".
Hertfordshire Assistant Chief Constable Nathan Briant said the two drivers had been identified and interviewed.
He said officers were "continuing to work with partners to fully understand the events" and the drivers had been "interviewed as part of the ongoing investigation".
"Whilst the events do occur regularly the meeting yesterday evening appeared far larger in nature than previous events, and it is now understood that an organiser had publicised the meeting on social media as a charity event," he said.
"Last night we identified more than 130 witnesses and an investigative team has been formed to ensure each of these is contacted to obtain their statements.
"We are also aware of a large number of people leaving the area prior to our arrival, among these are likely also to be further witnesses to the collision."
Police have asked witnesses to send footage of the crash to detectives.
Video footage shows one car passing another before the two collide and one strikes people standing at the roadside while the other hits spectators in the central reservation.
One witness said on Twitter: "I've just witnessed that horrendous crash in Stevenage, no more than 50ft away from me. I'm still trying to process it all."
Cruise-Herts planned the event on Thursday where people were due to gather to look at modified cars.
Organiser Rix Sidhu said he had been organising similar meets for 17 years and the latest was held to raise money for charity.
He estimated one of the cars that crashed was travelling at 60 or 70mph and then went into the crowd "at speed".
Mr Sidhu said: "We held the meet in a car park with a speed bump at the entrance. But unfortunately some people went a bit rogue.
"We try and stop that, we urge people...not to go out on the roads, not to risk injury or anything.
"But unfortunately, in this age of social media and Snapchat, people want to get footage and post things to their friends, which seems to drive some people to the main road."
In a statement on Facebook he said they would not be organising any more such events.
He also said the police were aware that the group met every Thursday evening, having attended in the past, and said the two drivers involved in the crash were "not regulars".
At the scene
By Adam Jinkerson, BBC News
Last night there were hundreds of people here, but this morning the only sign anything happened is an abundance of skidmarks in the adjacent car park - and police markings on the road at the scene of the accident.
There are yellow spray-painted markings that seem to indicate where the two cars involved were travelling, the point at which they met - and where they came to rest.
Some of those markings are on the pavement.
I can also see that first aid was given here.
There are are a few bits of medical paraphernalia left among the rubbish, which is now the only other evidence of how many people were gathered here last night.
Throughout the morning young people have been turning up to collect their vehicles but they have been too upset to talk about what happened.
Fellow organiser Dean Summerbee, 34, said people attending had been warned not to race or do wheel spins and burnouts.
He said: "It was horrific seeing it last night. It still plays over in my head in slow motion. I literally had to pull my mate out of the way.
"My thoughts go out to everyone who has been hurt. It's not something I'd like to relive again.
"I feel sorry for anyone who witnessed it. It was a nightmare last night."
The section of A-road passes a retail park near Stevenage Football Club.
Tom Adams, who lives in Welwyn Garden City and arrived shortly after the crash, said he knew the organisers "dotted all the Is and crossed all the Ts" and it was "not just a gathering of hooligans" but the event had been let down by a "bunch of boy racers".
He added: "There is a select group of people that have no consideration for other people and unfortunately that has come back to bite us."
Ciaran O'Connor, 33, was travelling home when he witnessed the crash which he described as "horrific".
Hertfordshire Fire and Rescue Service crews cut free one person trapped in a vehicle and provided "trauma care" to a number of injured people.
Stevenage Borough Council leader Sharon Taylor said such events were unauthorised and hard to regulate.
She said: "We will do whatever we can to make sure we don't get dreadful incidents like this. [But] it's not an easy thing to regulate."