A Pennsylvania state trooper is in good condition after taking a bullet to the shoulder during a morning highway chase that ended in a fiery crash on I-676 in Center City.

State trooper Patrick R. Casey, 31, continued firing on the suspect from his vehicle, even after being shot in the shoulder, said Pennsylvania State Police captain James Raykovitz of Troop K Philadelphia.

As of Tuesday evening, the shooter was in custody, another individual who was in the same car was arrested, and two more individuals who were in the car were being sought, with one still unidentified.

The shooting happened around 9:45 a.m. Tuesday on eastbound I-676 in the area of 22nd Street, following a traffic stop at around 9:30 over an expired registration sticker.

A field training officer who was traveling with a new trooper pulled over a two-occupant vehicle with four people inside it, Capt. Raykovitz said.

While one passenger was cuffed and two were sitting on the barrier, the fourth got back into the car and fled, sparking the chase.

Casey and other troopers followed the 17-year-old suspect, who has not been identified, westbound on 76 and then eastbound on 676.

Casey positioned his vehicle in front of the suspect's and was rammed. Then the suspect got alongside Casey's car and began firing at him, Raykovitz said.

While firing at Casey, the suspect crashed into a school bus, which was empty except for a driver, who was uninjured. 

"The suspect hit Trooper Casey's car, then lost control and went into the back of  a school bus,"  Raykovitz said.

"Upon impact with the school bus, the suspect jumped from the car and continued to fire at Trooper Casey, at which time he [Casey] began to return fire as he was seated in his car." 

Casey took a bullet to the shoulder, sustaining a flesh wound, and returned fire from inside his car, Raykovitz said. The shooter fled and was apprehended by troopers.

The suspect's car became engulfed in flame, which spread to the bus.

The shooter fled from the car and was apprehended by other state troopers, while the fire was brought under control. No injuries were reported other than Casey's bullet wound.

Andy Ocasio witnessed part of the incident from his nearby office window, he said. He saw the driver get out of his car after crashing into the school bus and flee under a nearby overpass.

"I saw the gentleman leaned against the car, there were cops everywhere with their guns drawn. He just started running," Ocasio said.

"I've never seen that many automatic rifles before. Thank god it was our guys who had them," he said.

RELATED: Two Philly cops hurt in early morning crash

The trooper who was wounded was likely shot through his windshield. That trooper's vehicle was clearly visible on 676 between 21st and 22nd streets with multiple bullet holes in the glass of the windshield.

A gun that looked like a black Glock, a clip, and shells were still scattered over 676 as state troopers secured the crime scene.

Gerald Spratt, 53, was waiting for SEPTA's 48 bus on 22nd Street when he heard gunshots, he said.

"I heard pop-pop-pop-pop-pop pop-pop-pop" he described. "I thought it was firecrackers. When I heard it again, I was like, 'That's not firecrackers. Then I came and saw the smoke."

"I'd just come from the foot doctor's. I couldn't believe it," said Spratt.

RELATED: Philly police ID officer shot in arrest raid

Gov. Tom Wolf said in a statement that he was relieved at news that the trooper is in stable condition. "Our men and women in law enforcement put their lives on the line each day to protect us, and we owe them a debt of gratitude. The shooter was swiftly taken into custody by the Pennsylvania State Police. No further details are available at this time.”

This is a developing story. Check back with Metro for updates.