Oakland police are to be the subject of a formal investigation after Iraq war veteran Scott Olsen suffered a fractured skull at an Occupy Oakland protest last week.

Oakland's Citizens' Police Review Board is launching the investigation after it received a complaint on Friday. Police in Oakland are bracing themselves for a general strike on Wednesday, which has been announced by the city's Occupy movement and is expected to cause disruption across the city.

Olsen, 24, was seriously injured after allegedly being hit on the head by a police projectile. He is still in hospital and unable to talk, communicating only through short written messages.

A source at the review board said the investigation will begin in the next few days, and is expected to last "several months".

"We're reviewing the information we have at the moment," the source told the Guardian.

The board received the complaint from a member of the public. The complaint "relates specifically to Scott Olsen", and was not filed by a member of Olsen's family.

An investigator has yet to be assigned to the case, but will be "within the next few days", the source said.

"I think it's a wonderful thing," said Alan Brill, who acts as a spokesman for Occupy Oakland.

"Just like every once in a while people do things that are wrong from our side, there is a small percentage of police that are out of control, and I'm glad they're being investigated."

Olsen, a former marine who served two tours of Iraq, was injured on Tuesday 25 October as police cleared the Occupy Oakland camp from its base at Frank Ogawa plaza, outside Oakland city hall.

Police used teargas and "less lethal" weapons to clear the plaza. Olsen was apparently struck in the forehead, knocking him to the ground. Video footage shows a police officer throwing an explosive towards a group of protesters who went to Olsen's aid.

More than 15 police agencies were involved in the operation that day, including San Francisco sheriffs.

There has been speculation on social media sites that it was a San Francisco sheriff who injured Olsen, with some Twitter and Facebook accounts naming an officer. However, sheriff spokeswoman Eileen Hirst said that while a platoon of 35 sheriffs did attend the Oakland operation, none of them were carrying teargas or less lethal weapons.

Hirst said the San Francisco sheriffs' involvement in the operation in Oakland was being reviewed internally, but none of the 35 officers who attended on 25 October had been suspended.

Thousands of Occupy protesters are expected to gather in Oakland for the general strike and mass day of action on Wednesday. The strike aims to "shut down" the city, culminating with a march to the Port of Oakland to prevent the transit of cargo.

"Oakland was the site of the last general strike in the US," said protester Tim Simons, at a press conference on Monday.

"On Wednesday, we're going to make history once again. We're going to make Oakland proud."

• This article was amended on 4 November 2011 to add the word "allegedly" in the following sentence: "Olsen, 24, was seriously injured after allegedly being hit on the head by a police projectile."