Ontario's police watchdog is investigating after a 30-year-old man who was Tasered by police died in a Greater Toronto Area hospital.

Peel Regional Police say they were called to Runningbrook Drive in Mississauga, Ont., around 3:15 a.m. for reports of a "suspicious male causing a disturbance."

Officers found the man in the backyard of a residence and, according to the Special Investigations Unit, "a struggle ensued."

"As part of the struggle, several use-of-force options were used, including the deployment of a conducted energy weapon," says Monica Hudon, SIU spokesperson.

Hudon said it's not yet clear what other techniques police may have used to subdue the man as the encounter escalated.

The man was taken into custody and soon after he lost consciousness. He was taken to a local hospital and pronounced dead shortly after 4 a.m., Hudon says.

SIU investigators have spoken to several neighbours who witnessed parts of the incident, she adds.

"We've gotten some good information this morning."

A neighbour who watched some of the encounter unfold from her home says she was awoken by police floodlights. Cruisers were driving up and down the street "looking for somebody."

CBC News has agreed not to name her because she fears for her family's safety.

She says she saw two police officers run into a dark backyard. Shortly after, the two officers came running back to the street. As that was happening, more police and ambulance arrived at the scene.

A man was carried out of the backyard on a stretcher and paramedics began administering CPR to him, she says.

Another neighbour who identified himself only as Roman says that shortly before police arrived, there was a group of people on the street yelling.

A neighbour says she saw two police officers run into a dark backyard. Not long after, paramedics arrived and carried a man out of the yard on a stretcher. (Jeremy Cohn/CBC)

This is the second stun-gun death in Mississauga in recent months.

The SIU is also investigating the death of a 34-year-old man who died after police used a stun gun on him on Sept. 10.

In that case, the watchdog said Peel police officers responded to a domestic call in the Malton, Ont., area around 9 p.m.

The SIU said the "officers became involved in an interaction with a man which involved the use of a conducted energy weapon."

The agency said the man stopped breathing, his heart stopped beating and he was later pronounced dead in hospital.

The SIU is a civilian oversight agency that invokes its mandate when a police interaction leads to death or serious injury.