Hundreds of New York City police officers began entering Columbia University on Tuesday evening, as anti-Israel protesters continue occupying a campus building.

Dozens of Columbia protesters occupied Hamilton Hall earlier Tuesday and blocked the entrances to claim the area as their own. Protesters could be seen smashing windows and barricading themselves inside the building, one day after the university began suspending students for refusing to leave a pro-Palestinian encampment on campus.

The New York Police Department received a letter from Columbia authorizing its officers to move onto campus. The NYPD closed off streets around Columbia and warned the protesters to leave Hamilton Hall. Columbia students were previously ordered to shelter in place, according to an email late Tuesday.

“Shelter in place for your safety due to heightened activity on the Morningside campus. Non-compliance may result in disciplinary action. Avoid the area until further notice,” Columbia’s Emergency Management Operations Team told students.