District police are investigating two arson fires that occurred Monday night and early Tuesday in Northwest Washington, according to a department spokesman.

The first fire was reported about 9:40 p.m. in the 600 block of T Street NW, near the Shaw-Howard University Metro station. Police said a man was seen piling cardboard boxes into a portable bathroom near a building under construction and then lighting the boxes on fire.

Officer Araz Alali, a police spokesman, said the fire spread to the building but was quickly extinguished. The man was not found.

About 1 a.m., Alali said, another fire broke out in the basement of an apartment building in the 1400 block of Oglethorpe Street NW, near Brightwood. Police believe that someone poured a flammable liquid that started the fire, Alali said.

Firefighters extinguished it before it spread through the rest of the building, police said, but 18 tenants had to be temporarily relocated because they lost power.

No arrests have been made in either case; police said they could not say whether the fires were related. The addresses are 31 / 2 miles apart.