An investigation by Ontario's police watchdog into a fatal shooting that occurred in the drive-thru lane of a Burger King restaurant in Toronto could take a month or longer to complete.

The shooting took place Sunday evening at the Burger King near the corner of Keele Street and Wilson Avenue.

The Special Investigations Unit (SIU) says police officers were attempting to make an arrest at about 9 p.m. on Sunday when "there was an interaction between police and a man, and a short time later, the man sustained a gunshot wound."

The man was pronounced dead at Sunnybrook Health Sciences Centre. The SIU says he was in his 20s.

The CBC's Jeff Semple reported that unmarked cruisers had blocked both sides of the drive-thru and trapped a vehicle in there just before officers moved in.

SIU investigators remained at the scene through the night after the shooting.

Jasbir Brar, a spokesperson for the SIU, said five investigators and three forensic investigators have been assigned to the case.

Brar said the SIU typically tries to close its investigations "within 30 business days," though an investigation involving a shooting is often more complex.

Adeleke Adewsi was at the bank next door when he heard the gunshot.

"I was scared," said Adewsi, who wanted to know immediately what was happening.

David Toste lives in a building nearby and heard "a bang" when he was watching TV on Sunday evening.

He said he couldn’t see the shooting from his vantage point, but he watched several plainclothes officers arrest another man and a woman.

"It looked like they had one guy behind one of the vehicles and it looked like they were trying to search him," Toste said.

The CBC's Linda Ward reported that other witnesses described seeing several plainclothes individuals surrounding the car in the drive-thru.

"That car was towed earlier today, blanketed to preserve evidence," Ward said.

Anyone with information or who may have witnessed the shooting is asked to contact the SIU at 416-622-1886 or 1-800-787-8529 extension 1886.

The SIU investigates all reports involving police where there has been death, serious injury or allegations of sexual assault.

With files from the CBC's Jeff Semple, Tony Smyth and Linda Ward