Only one weapon was used, a 9 mm Glock handgun that was registered in Moseley's name.
Moseley was once a security guard at the facility where the shooting occurred, a law enforcement official said.
Four injured people were transported to Johns Hopkins Bayview Medical Center in Baltimore with gunshot wounds needing urgent attention.
A spokesman for a health system that includes Christiana Hospital in Newark, Delaware, said one patient was being treated there. The Washington Post couldn't immediately confirm that number.
Witnesses say they are seeing multiple police departments and ambulances responding.
Sen. Benjamin L. Cardin (D-Md.) on Twitter called the situation "just horrific", adding, "I'm monitoring this carefully, and urge residents to stay clear of the area until further notice". "The State stands ready to offer any support", he tweeted.
Gahler said the suspect also had several magazines and there no shots fired by law enforcement officers.
"FBI Baltimore is responding & assisting @Harford_Sheriff with an Active Shooter situation". Please refresh for the latest updates and information.
The facility has almost 1,000 employees, company spokesman Pete Strella said, and processes products, including pharmaceuticals, for delivery to more than 2,500 stores.
The centre processes products, including pharmaceuticals, for delivery to more than 2500 stores.
The shooting sent survivors screaming and running in all directions from the Rite Aid distribution center in northeastern Maryland.
"The shooting happened adjacent to the primary building", she said. He said people were shot both inside and outside the Rite Aid warehouse.
According to the sheriff's office, a total of seven people were injured and 4 have since died.
Police say the gunman who was killed by officers after shooting four people at his Wisconsin workplace on Wednesday is a 43-year-old man who had been employed at WTS Paradigm, a software company, since April 2017. The Harford Fire Department tweeted that there were "multiple victims" and that local police were "searching for suspect".