(CNN) -- A man walked into a Pittsburgh psychiatric hospital with a pair of guns Thursday and began firing, leading to injuries to seven people and two deaths -- including his own as police arrived and shot back, authorities said.
"This is a tragic day, a sad day, a senseless day," Mayor Luke Ravenstahl said after the shooting at the University of Pittsburgh Medical Center's Western Psychiatric Institute and Clinic.
City and hospital officials speaking at a Thursday evening press conference did not give the identity of the shooter, nor did they elaborate on what his motive might have been.
The assailant walked through the psychiatric hospital's front door with a pair of semi-automatic handguns around 1:40 p.m., according to Ravenstahl.
Officers from the University of Pittsburgh police department arrived within "a matter of minutes" and "engaged" the gunman, the mayor added.
"There is no doubt that their swift response saved lives today," said Ravenstahl, lauding the police officers' "courage and willingness to step up."
Like the man who is believed to be the shooter, no details were offered on the other person killed besides the fact he, too, was an adult male.
Those wounded were swiftly ushered to nearby UPMC Presbyterian. All are expected to survive.
Officials said two of five hospitalized patients have undergone surgery. The conditions of the five ranged from fair to serious.
Two victims were treated and released. One had an injury not related to a gunshot wound.
Medical center officials said the injured included hospital employees and a visitor.
"I expect all of these patients to survive and do well," said Dr. Donald Yealy, chairman of emergency medicine at UPMC, said in a statement. "As the busiest trauma centers in the region, we prepare for this kind of multi-casualty situation."
Ravenstahl said that a University of Pittsburgh police officer "was grazed with a bullet, and I believe he is doing well."
Steven Bartholomew, a spokesman for the federal Bureau of Alcohol, Tobacco, Firearms and Explosives, said that his agency is "working with Pittsburgh police and other authorities in the investigation."
"Any firearms recovered will be traced to determine their source," Bartholomew said.
Claudia Roth, the Western Psychiatric Institute's CEO, said five of those hurt are hospital employees. No other details were offered on the victims, including whether they had been targeted, how they got hurt and who might have shot them.
At one point Thursday, the University of Pittsburgh's official Twitter account had a message that suggested a "possible 2nd actor" was involved and called for a lockdown.
But later in the day, the medical center's own Twitter feed had a message insisting "there was no second shooter" and that there was "no hostage situation," as had been reported elsewhere.
While the crisis appeared to be over by 6 p.m., Pittsburgh's mayor stressed the hospital -- particularly its first and second floors, where the gunman may have been -- "is still an active scene." That said, authorities said they have no indication that anyone else was involved or that there is any continuing danger.
"Our crews are methodically going through the building to make sure that every floor is cleared and every room is cleared," Ravenstahl said.
The University of Pittsburgh School of Medicine's psychiatric department is based at the institute, which describes itself on its website as a "leader in the treatment of mental health and addictive disorders."
U.S. News and World Report ranks University of Pittsburgh Medical Center -- and thus the Western Institute -- as being the nation's eighth-best psychiatric hospital.
In addition to about 2,000 employees, the hospital has 310 beds, of which about 90% had been occupied on Thursday, said Roth.
She emphasized after the shooting that treatment at the facility "will be uncompromised," with care continuing to be delivered through the day and night "the same way we always have."
CNN's Susan Candiotti contributed to this report.