The U.S.’ Defense Intelligence Agency says it's investigating a potentially harmful substance sent to a U.S. military base in Washington.

The agency said that a suspicous substance was detected during routine mail screening at Joint Base Anacostia-Bolling on Tuesday, local time. The initial tests discovered that the substance was a possible biological toxin.

The DIA declined to confirm whether the substance may be the deadly poison ricin. The agency said that it will not comment further on the issue until an investigation is completed. It explained that prudent screening methods and force protection measures were implemented to keep base personnel from being harmed.

In the wake of the Boston Marathon bombing, poison was found in letters sent to President Barack Obama and Republican Senator Roger Wicker of Mississippi. A suspicious package was also delivered to a naval facility in Arlington, Virginia.