WELLAND, Ont. -- The province's police watchdog is investigating after a 21-year-old woman died following a crash in Welland early Friday morning.

The Special Investigations Unit says Niagara regional police received a call regarding a single-vehicle collision at about 1 a.m.

When officers arrived at the scene, the SIU says the woman fell face down to the ground and was "bleeding profusely."

The SIU says she was transported to hospital where she was pronounced dead.

The agency did not elaborate on the interaction the woman had with police.

It says three investigators and two forensic investigators have been assigned to the case.

The SIU is an arm's-length agency that investigates reports involving police where there has been death, serious injury or allegations of sexual assault.