The shooting happened Tuesday around noon near Fond du Lac and Hampton avenues, according to the Milwaukee County Transit System. Police said two 18-year-olds were shot and are expected to survive. Crime scene tape surrounded the bus, which was riddled with bullet holes.

Michael Brown, vice president of Amalgamated Transit Union Local 998, said the violence broke out when two people inside the bus struggled over a gun and shot each other.

"Those individuals was shooting on the bus and could have shot some innocent people on the bus," Brown said. "There were two individuals inside the bus wrestling for the gun and shot each other."

Bullets flew from inside and outside the bus and came close to the operator and passengers, who police say were not involved. Brown said the operator watched the shooting unfold at close range.

"It took a mental impact on our operator, watching that happen right in front of his face, and you don't know where the bullets are going," he said.

Brown said the county has already taken steps to protect drivers but needs to do more. He called on lawmakers to give MCTS security officers additional authority and tools.

"We have kids on the bus, people going to work, so we just want to make the system better, safer, and the only way we can do that is if they pass a law to give MCTS security more tools to do their job," Brown said. "We want the same as well to protect the citizens of Milwaukee on the bus and bus operators so they can do their job safely."

MCTS said it is working to enhance security, including launching a safety program this year with more than 3,500 cameras on buses and providing additional safety training to drivers. The agency said 21 public safety officers ride buses across the county, and new full-sized safety shields will be installed on 70 buses next year to help protect operators.

In a statement, MCTS said: "It's deeply concerning when community issues affect our public transportation system. MCTS does not tolerate acts on our buses, or anywhere in our community, that threaten the safety of others. The safety of our riders and employees is our top priority."

The shooting is the latest flashpoint in a broader debate over how to keep Milwaukee's public transit safe for the workers who operate it and the thousands of residents who rely on it daily. Brown said the union wants more security personnel on buses, arguing that cameras and shields alone cannot prevent confrontations from escalating.

The two wounded teenagers were taken to a hospital, and police have not announced any arrests. The investigation is ongoing.