Google's CEO Sundar Pichai has sent a note to all staff in the company following a shooting at YouTube's headquarters.

Three people were shot and one person is dead following an active shooter at the company's offices in San Bruno, California. There was a fourth victim with non-shooting injury.

In his note, which was tweeted by Google's communications team, Pichai assured employees that the situation was now under control, and thanked first responders to the incident, as well as for the outpouring of support.

"I know a lot of you are in shock right now," he wrote. "Over the coming days, we will continue to provide support to help everyone in our Google family heal from this unimaginable tragedy."

Here's the note in full:

Everyone,

Earlier this afternoon, while our employees were having lunch, we received reports of an active shooter at YouTube in San Bruno. Law enforcement and our Security team worked to evacuate the buildings and prioritize the safety of everyone there.

The best information we have is that the situation is contained. We are very actively monitoring and working with local authorities and hospitals. It's with great sadness that I tell you — based on the latest information — four people were injured in this horrific act of violence. We're doing everything we can to support them and their families at this time.

I'm grateful to everyone inside and outside the company for the outpouring of support and best wishes. I am especially thankful to the first responders and our own security team who acted so quickly to keep people safe.

I know a lot of you are in shock right now. Over the coming days, we will continue to provide support to help everyone in our Google family heal from this unimaginable tragedy.

Let's everyone come together now to support Susan and the YouTube team.

—Sundar

In a tweet on his personal account, Pichai pledged his support for employees and the YouTube community following the tragedy.

There are no words to describe the tragedy that occurred today. @SusanWojcicki & I are focused on supporting our employees & the @YouTube community through this difficult time together. Thank you to the police & first responders for their efforts, and to all for msgs of support. — Sundar Pichai (@sundarpichai) April 3, 2018

YouTube CEO Susan Wojcicki also tweeted, where she said there were "no words to describe how horrible it was to have an active shooter" at the company's headquarters.

There are no words to describe how horrible it was to have an active shooter @YouTube today. Our deepest gratitude to law enforcement & first responders for their rapid response. Our hearts go out to all those injured & impacted today. We will come together to heal as a family. — Susan Wojcicki (@SusanWojcicki) April 4, 2018

Other major tech executives have turned to Twitter to post their messages of support, including Twitter's Jack Dorsey, Uber's Dara Khosrowshahi and Apple's Tim Cook.

UPDATE: April 4, 2018, 11:24 a.m. AEST Added comment from YouTube CEO Susan Wojcicki.