Ten people were killed and 10 more wounded after a 17-year-old carrying a shotgun and revolver opened fire at the high school about 30 miles from downtown Houston.
According to Harris County Sheriff Ed Gonzalez, nine of the victims were students. Texas Gov. Greg Abbott called the shooting "one of the most heinous attacks that we've ever seen in the history of Texas schools."
According to police officials, the 17-year-old is in custody and has been charged with capital murder and a second person has been detained.
Shortly after the shooting, Watt tweeted, "Absolutely horrific."
Watt has been active in the surrounding community since he was drafted by Houston in 2011. Most notably, he started a fundraiser last August that raised more than $37 million for those affected by the flooding from Hurricane Harvey.
The Texans also released a statement.
"On behalf of the Texans organization, we are saddened by the tragic events at Sante Fe High School this morning and extend our thoughts and heartfelt condolences to the victims, their families and all those affected. We are grateful for the brave first responders, law enforcement officials and medical personnel. The Texans family will continue to pray for our neighbors."