Police on Wednesday were investigating the shooting, which occurred a day earlier at about 1:30 p.m. in Leonardtown in southern Maryland.
"There are many questions we do not have the answers to at this point," State Police Superintendent Woodrow Jones told a news conference on Tuesday.
The trooper, whose name was not immediately released, was responding to two 911 emergency calls about "a guy acting suspicious" and possibly armed with a gun in a residential neighborhood. He arrived to find the teen in a home's driveway "in a shooting stance, pointing a gun at the trooper," Jones said.
"The trooper fired at the male, wounding him," Jones said.
The teen, who was identified as Peyton Ham, 16, and who lived a few houses away, then pulled a knife, Jones said.
"The trooper ordered him to drop the knife before he fired again," Jones said.
The teen was rushed to a nearby hospital where he was pronounced dead, he said.
Investigators later determined the teen was carrying an airsoft gun, a toy that is a close copy of an actual weapon and used to shoot BBs, which Jones described as "a replica gun that represented a real weapon."
Police recovered both the airsoft gun and the knife from the scene, he said.
Following police protocol, a criminal investigation into the incident is underway and results will be turned over to the St. Mary's County state's attorney to determine whether charges are warranted.
The trooper, who has been on the force for two years and 7 months, was placed on administrative leave, Jones said. He extended condolences to the teen's family.
"Any loss of life is a tragedy. I can't even comprehend how sad this can be," Jones said.