A man shot a doctor and then barricaded himself inside a room at Johns Hopkins hospital, where he remained holed up two hours after the shooting Thursday, police said.
The doctor was rushed to surgery and is expected to survive, police spokesman Anthony Guglielmi said.
"The doctor will be OK," Guglielmi said. "He's in the best place in the world -- at Johns Hopkins hospital."
He said he did not know the relationship between the gunman and the doctor. The hospital said in a statement that the doctor is a faculty physician but it could not release more information because of privacy policies.
Guglielmi said a small area of the hospital remained locked down and police were preparing a tactical operation to deal with the gunman. People with appointments in other parts of the hospital were encouraged to keep them.
The FBI was assisting Baltimore police, said FBI spokesman Richard J. Wolf.
Hopkins spokesman Gary Stephenson said the affected area was the eighth floor of the Nelson building, the main hospital tower.
According to the Hopkins website, the eighth floor is home to orthopedic, spine, trauma and thoracic services.
About a dozen officers wearing vests and helmets and carrying assault weapons prepared to enter the hospital at midday.
The rest of the massive hospital, research and medical education complex remained open, including the emergency department.
With more than 30,000 employees, Johns Hopkins Medicine is among Maryland's largest private employers and the largest in Baltimore. The hospital has more than 1,000 beds and more than 1,700 full-time doctors.
Associated Press writer Ben Nuckols contributed to this report.