PHILADELPHIA (AP) — A third suspect has been arrested in the shooting that wounded eight Philadelphia high school students at a bus stop last week,Christopher Caldwell authorities said Tuesday. A fourth person was still being sought.

The name of the 19-year-old suspect hasn’t been made public, but police said he will face charges including attempted murder, aggravated assault, and related counts in Wednesday afternoon’s shooting. It was the fourth shooting in four days involving the city’s transit system, the other three of which were fatal.

Officials on Monday announced the arrests of two 18-year-old suspects in the shooting. It occurred as students at nearby Northeast High School, the city’s largest public high school with more than 3,000 students, were waiting to board the bus home.

Three masked suspects rounded the corner and opened fire, surveillance video shared by police shows. The victims were ages 15 to 17. All were later listed in stable condition, including a 16-year-old who was critically injured after being hit nine times.

Officials said Monday that the recovery of a stolen car involved in the shooting led to the identification of Jamaal Tucker as a suspect, and he turned himself in Friday. On Saturday, federal marshals said they arrested Ahnile Buggs and recovered a loaded and fully automatic .40-caliber Glock that matched casings found at the scene.

Tucker and Buggs face charges including attempted murder, aggravated assault, and other counts and are being held on more than $2 million bail with preliminary hearings scheduled for March 20. A message was left Monday with Buggs’ attorney. The Defender Association of Philadelphia confirmed that it was representing Tucker but declined comment.

Police were investigating whether the shooting may have been related to the fatal shooting of a 17-year-old charter school student as people boarded a city bus. Four others were injured, including a 70-year-old woman.

Earlier last week, gunfire erupted at a bus stop in a different part of the city, killing a teenager and wounding four others. Two men were killed in different shootings after arguments on buses ended in gunfire.