The Penn North subway station was closed for three hours last night as authorities investigated the explosion of two bottles filled with an acidic substance, a Maryland Transit Administration spokesman said.

MTA police initially responded before 7 p.m. to a report of a suspicious package found on the station's platform level, said the spokesman, Richard Solli. They determined those items were not a threat, but as they were leaving the station, they heard two loud pops that sounded like a car backfiring about 7:10 p.m., Solli said.

Authorities think that two bottles, 20 feet apart on the mezzanine level, were thrown near an escalator in the station, he said.

Passengers were evacuated from the station about 7:30 p.m.; the station reopened at 10:30 p.m., Solli said.

No passengers were injured. The station attendant complained of dizziness and was taken to a hospital for treatment, the spokesman said.

The city police arson unit will investigate the incident, including surveillance video, Solli said.