Police were called just after 9am on Thursday morning to reports of a shooting at the junction of Spesutia Road and Perryman Road in Harford County.
No confirmation has been made on the number of victims or if the shooter is in custody.
Gahler said Thursday there is no longer any threat at the site at a brief midday news conference.
The facility packages pharmaceuticals and other products for delivery to more than 2,500 stores.
"Multiple victims. Please avoid the area", the sheriff's office tweeted", after the shooting.
Gahler said Mosely once was a security officer but not at the Rite Aid distribution center.
Johns Hopkins Bayview Medical Center said in a statement Thursday afternoon that it was waiting to confirm that family members have been notified before sharing any additional information.
"We are closely monitoring the horrific shooting in Aberdeen".
Mourinho confirms two players to miss Wolves clash through injury
Jose Mourinho has said three straight wins has not changed his opinion that Manchester United are in for a tough season. A team that yesterday looked very bad, today looks very good and yesterday looked very good today looks very bad.
Police have named the woman who killed herself after fatally shooting three people, and injuring three others, at a warehouse in the USA state of Maryland.
The shooting in Harford County comes just one day after a shooting at a software company in Middleton, Wisconsin, that left four people injured.
Alexi Scharmann told WBAL-TV that her mother, who works inside the Liberty building at the distribution center, texted her about a shooter inside the facility.
The suspect used a single handgun and no shots were fired by law enforcement officers. He went on, "our prayers are with all those impacted, including our first responders".
The Rite Aid centre, part of a larger warehouse in an industrial park, has almost 1,000 employees, company spokesman Pete Strella said.
A 26-year-old woman open fired in a warehouse in the U.S. state of Maryland, killing three people and wounding several others before taking her own life, according to authorities and a witness account. Inside, where there were about 65 people, she fatally shot Giri and Reyes and also shot Mitchell, Villegas and Purna, who survived.
Others said they helped the wounded, one person tying blood-soaked jeans around a man's injured leg in a bid to stop the bleeding.
The Bureau of Alcohol, Tobacco, Firearms and Explosives sent agents from Baltimore.
The attack comes almost three months after a gunman shot dead five staff members of The Capital Gazette, in Annapolis, Md., 59 miles from the Rite Aid distribution center.