Police say they are trying to piece together what was happening in the life of a Melbourne family before a fatal car crash on Wednesday afternoon that killed three children.

A mother and her four children were dragged from their car after it plunged into Lake Gladman in Wydnham Vale in Melbourne’s west, but little is known about what caused the crash.

Passersby and police officers jumped into the lake and smashed holes through the vehicle’s windows to rescue the four trapped children, each aged under six.

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Three children died – one at the scene, the second on the way to hospital at 5.30pm, and the third in hospital before 7pm, police said.

The fourth child remains in hospital in a serious condition, while the 35-year-old mother is currently being questioned by homicide police.

Speaking to media on Thursday morning, police appealed for members of the local Sudanese community to come forward with more information about the mother of seven and her family.

“We need to understand what led to this, we need to understand the background of family, we need to understand what was happening in their lives and we certainly need to understand what their movements were yesterday,” a spokesperson said.

“We believe there are people out there who will know.”

Victoria Police Superintendent Stuart Bateson said it was unclear what caused the “tragic” crash.

“What we do know is that when the car [crashed] passersby and police members got some of the children out and did CPR,” he said.

“It’s going to be a really harrowing investigation. It’s really tough, it’s tough on everyone involved.”

Aerial pictures from the scene showed a vehicle submerged in water about 10 metres from the lake’s edge.

The car has remained in the lake overnight and will be retrieved on Thursday.

A witness said a man had tried to rescue the trapped children and pulled out a boy for paramedics to try and save him.

“Travis jumped into the water and smashed the back screen of the car and got one little boy out,” a woman called Michelle told Fairfax Radio.

“It’s just a horrible scene.”

A woman who lives opposite the lake said she saw firefighters pull a child from the vehicle.

“They couldn’t get the car open,” she told Fairfax. “They were banging, trying to get the car open and trying to smash its windows.”

She said after the child was pulled out, paramedics conducted CPR for some time. The woman said the incident had obviously taken an emotional toll on all involved, particularly the firefighters.

“They looked very emotional, they were exhausted,” she said.

Dozens of emergency workers attended the scene to treat the injured.

Police and emergency workers have placed a wide cordon around the vehicle.

Other residents noted that Manor Lakes Boulevard, adjacent to the lake, was known to be a speeding and “hoon” hotspot, while neighbouring streets were regular drag strips.

“Cars travel down here so fast,” Rebecca Reschke told Fairfax.

—with AAP/ABC