(Reuters) – Staff members returned for the first time on Friday to the Florida high school where 17 students and faculty were gunned down by an ex-student with an assault rifle last week in one of the deadliest school attacks in U.S. history.

Teachers were welcomed back to Marjory Stoneman Douglas High School in Parkland as part of what the school district called an “emotional and difficult recovery process.”

Staff could be seen arriving at the school in their cars on Friday, passing perimeter checks guarded by police cars, according to video from Miami’s 7News.

Classes are due to resume on Wednesday. On Sunday, students and their parents are invited to come to the campus for “support services,” the Broward County Public Schools district said in a statement.

Nikolas Cruz, a 19-year-old former student of the school, has been charged with 17 counts of premeditated murder in the assault.

Many of the student survivors of the massacre have since advocated for tougher gun-control laws. They have been widely interviewed on national television and have traveled to meet politicians in Tallahassee, the state capital, and U.S. President Donald Trump in the White House.

(Reporting by Jonathan Allen and Gina Cherelus in New York; Editing by Colleen Jenkins and Jonathan Oatis)