WASHINGTON – Metro Transit Police increased patrols and conducted additional sweeps with police dogs on Tuesday, but a number of riders said they didn’t notice the changes.

Bombs exploded Tuesday at a Brussels airport and in the city’s subway, killing at least 36 people and wounding dozens. The Islamic State group claimed responsibility for the attacks.

When a similar question was raised last month following a shooting on a Green Line train at the Anacostia Station, Metro Transit Police Chief Ron Pavlik said it could come down to which station entrance you happen to use, or where you stand on platforms.

“You could be on an eight-car train, and an officer’s on the sixth car while something happens on the first car, so officers are out there riding trains, they are on platforms, they are on mezzanines,” Pavlik said.

Metro Transit Police are authorized to have 479 sworn officers, but Pavlik estimated last month the department only has about 440.

The department, like several other police departments in the region, has faced vacancies for several years as officers retire and recruitment processes drag on.

Transit Police officers who patrol the system work in shifts to cover Metro facilities across 1,500 square miles — including Metro’s 91 stations, Metro tracks and Metrobuses.

“[P]utting an officer in every car, you know at rush hour we’re averaging 900 rail cars, so that’s just not a realistic expectation,” Pavlik said last month.