According to the Baltimore Sun, county police chief Terrence Sheridan told reporters Monday evening that police were searching for four people in connection with the incident.
The officer, whose name hasn't been released, was shot around 2 p.m.in the Perry Hall area.
Vinson says officers recovered the suspects' Jeep, found ditched on a road.
Across the country, Sheridan said, the number of police officers killed in the line of duty has increased.
" In 2000, Officer John Stem died of complications relating to a duty gunshot wound he suffered in July 1977".
The officer was not immediately identified.
Just before 2 p.m., the officer responded to a burglary call in the unit block of Linwen Way.
Authorities were advising motorists to avoid the area and residents to shelter in place while the hunt for the suspect or suspects continued.
Police searched the area for the suspect or suspects involved with the officer's death and the manhunt will continue through the night.
At this time, police are not able to confirm if the officer was shot. We're seeing police officers assaulted. So we are telling everybody - be on your toes, don't be complacent, work with each other.
"What exactly happened, we are not sure yet until an autopsy is performed", Vinson said at a news conference Monday.
"It's bad, we're seeing something in this country today that we've never seen before".
About an hour later Baltimore County Police & Fire posted on social media saying they were searching for an armed suspect. "We are proud of the bravery and courage and sacrifice of all the men and women who serve on our police force". CBS reports that the officer was a female and was investigating a report about suspicious vehicles when she was killed on Belair and Klausmier.
Volunteer firefighter Logan Kurek said he ran outside and performed CPR on the officer until paramedics arrived, and says her injuries were consistent with being run over by a auto.
Amendt said his wife also ran outside and tried to revive the officer.
Parents from three elementary schools - Perry Hall, Seven Oaks and Gunpowder - were placed on lockdown, with parents told they couldn't pick up their children after classes let out.
Students at three local elementary schools were held over on alert status for the entire afternoon because of police activity.
Information from the Associated Press was used in this report.