THE PERPETUALLY congested lanes of the Vine Street Expressway were transformed into a terrifying hellscape yesterday morning, as flames devoured a school bus' skeleton and Pennsylvania State Troopers exchanged gunfire with an armed suspect.

The hair-raising episode began with a routine car stop over an expired registration tag, and ended with Trooper Patrick Casey taking a bullet to the shoulder - and a 17-year-old alleged gunman behind bars.

The teenager, identified Wednesday morning as Giovanni Cotto, was charged as an adult last night with attempted murder, aggravated assault, assaulting a law-enforcement officer and related offenses.

State Police Capt. James Raykovitz ran through the bizarre sequence of events during an afternoon news conference in front of the Troop K barracks where Casey has spent his six-year law-enforcement career.

A pair of state troopers, William Evans and Timothy Jeter, pulled over a car with an expired registration tag about 8:55 a.m. on Interstate 76, near the University Avenue exit.

They called for backup - which arrived in the form of Casey - and asked the four male occupants to get out of the vehicle for reasons that were unclear yesterday, Raykovitz said. A 19-year-old man was handcuffed and loaded into a police cruiser.

And that's when all hell broke loose. Raykovitz said the 17-year-old jumped back into the car and sped off, and the three troopers gave chase, leaving behind the two other occupants, who fled the scene.

The pursuit wound through the westbound lanes of I-76 before crossing over to Interstate 676. "At some point during the pursuit, Trooper Casey went around the suspect vehicle and tried to box him in, or do an induced stop, as we call it," Raykovitz said.

The young driver, police said, started to fire at Casey. He allegedly rammed into Casey's cruiser, causing him to lose control and slam his car into the back of a school bus, igniting a massive fireball on the expressway near 21st Street that would have been right at home in "Mad Max: Fury Road."

Raykovitz said the shooter got out of his smoldering vehicle and allegedly continued to fire at Casey, wounding him once in the left shoulder.

Casey, 31, returned fire from his cruiser, apparently shooting back at the gunman through his windshield. Several other state troopers also fired at the gunman, who was not wounded.

Casey was listed in good condition at Hahnemann University Hospital, but the bullet was still lodged in his shoulder.

Raykovitz said three state troopers and two plainclothes Philadelphia police officers arrested the alleged shooter without incident.

The school bus was empty except for its driver, a retired firefighter who escaped without injury. No motorists were injured, but traffic in the area was snarled for hours.

Before the crash, the bus had dropped off students at St. Katherine Day School, a special-education school in Wynnewood run by the Archdiocese of Philadelphia, according to School District of Philadelphia officials.

State Police said last night that the 19-year-old man who was taken into custody in the morning was later released, and is not facing any charges.

One of the two occupants who fled when the troopers chased after the gunman met with investigators last night. It was unclear if he would face any charges, or if the other occupant was also planning to meet with police.

Shortly before Raykovitz spoke about the case yesterday afternoon, Hector Cotto paced in front of Troop K's sprawling headquarters on Belmont Avenue, pausing every so often to peer through a wrought-iron fence at an empty State Police SUV.

Cotto said his 17-year-old son, Giovanni, had been involved in the dramatic incident, but he was unsure at that point to what extent.

"Do you know if they're going to bring them out over here?" Cotto asked a reporter, glancing toward the SUV.

Cotto said a detective called him yesterday after his son, who lives in North Philadelphia, had been arrested.

"We came here, but they didn't tell us nothing," Cotto said.

"He's a good kid," Cotto said. "But if they did something wrong, they have to pay for it."

He said his son, a junior in high school, had been arrested once before on a drug charge. "In our neighborhood, [drugs] are everywhere."

A number of elected officials, including Gov. Wolf, praised Casey's bravery and expressed relief over his condition.

Raykovitz said Casey, a Bucks County resident, was surrounded by his friends and family at Hahnemann. "I respect and admire the way Trooper Casey responded to being shot at," he said.

Pennsylvania State Troopers Association president Joseph Kovel issued statement that read: "This morning we are reminded of the sacrifice police officers are ready to make every time they put on their uniform. Our wounded brother is a hero, and we're relieved to learn he will recover from his injuries."

- Staff writers Matt Gelb

and Kristen A. Graham

contributed to this report.

On Twitter: @dgambacorta