A bus which ploughed into a crowd of people killing four, failed to stop at a pelican crossing, police have said.
The bus was on a regular run
Officers from Lincolnshire are now looking at whether mechanical failure or driver error was to blame for the incident near the town of Ingoldmells.
The victims were Richard Rhodes, 32, his wife Paula, 30, from Mansfield, Notts, a boy aged five and his mother.
A man aged 41 and a four-month-old baby, thought to be the woman's husband and child, are both critically ill.
The injured man and baby were taken to Pilgrim Hospital in Boston, but the baby was later transferred to Queen's Medical Centre in Nottingham.
Mr and Mrs Rhodes and the five-year-old boy, to whom they were not related, were all pronounced dead at the scene.
The second woman, who was aged around 30 and is believed to be the boy's mother, died later in hospital after being trapped under the bus.
A number of other people suffered minor injuries and shock.
Police Inspector Les Parker said: "There is an awful lot of work to be done to establish what happened.
"At this stage, it is clear that for some reason, the bus failed to stop on the approach to a pelican crossing and
collided with a BMW car and people crossing the road.
"It is too early to say why, but we are looking at a possible mechanical
defect and/or driver error."
He added: "This incident shows how a pleasant day at the coast can
so quickly end in tragedy.
"Our thoughts and sympathies are with those close to the deceased and the
Police have interviewed the driver of the Volvo bus, a 50-year-old man from
Louth, as well as a number of witnesses.
The bus, owned and run by Lincolnshire RoadCar Ltd, was on a regular service
between Chapel St Leonards and Ingoldmells.
The accident happened outside the car park near the Fantasy Island pleasure
park at around 1700 BST on Sunday.
Police Inspector Dick Holmes later dismissed suggestions that cars parked on
double yellow lines near Fantasy Island had played a part in the collision.
He said: "So far, from all that we know, the parking of cars on Sea Lane made no
contribution to this tragedy whatsoever."
John Woodward, the chairman of Blue Anchor Leisure, which runs the theme park,
said he had expressed concerns about illegal parking to police five times over
He said: "We have a massive traffic problem outside Fantasy Island, with motorists
parking on double yellow lines and the grass verges,"
"The parked cars cause congestion and they blind oncoming traffic. If someone
was crossing the road, you'd have trouble seeing them."