In the quiet borough of West Mifflin, Pennsylvania, a night of camaraderie turned deadly, leaving one man dead and another facing serious charges. On Saturday, August 16, 2025, Allegheny County Police responded to a welfare check at a home on the 1100 block of Thompson Run Road, just before 11 p.m. What they discovered inside the residence was a scene of violence that would shake the community.
First responders found 20-year-old Andy-Steven Nimajuan-Sitan with deep lacerations on both hands, his condition serious but stable. Nearby, another man lay lifeless, riddled with multiple wounds. The Allegheny County Medical Examiner later identified the deceased as Mario Alejandro Perez Santos. The discovery marked the beginning of a somber investigation into a fatal confrontation that unfolded within the walls of the unassuming home.
According to the criminal complaint, Nimajuan-Sitan shared a chilling account with investigators. He and Perez Santos had spent the day drinking together, their hours filled with the kind of camaraderie that often defines close friendships. But as the night wore on, the mood shifted. A dispute erupted, escalating with terrifying speed. Nimajuan-Sitan claimed that Perez Santos, in a moment of aggression, drew a knife and attacked him. In the chaos that followed, Nimajuan-Sitan fought back, leading to the fatal outcome that claimed Perez Santos’ life.
Nimajuan-Sitan, bloodied and shaken, was rushed to a local hospital for treatment of his injuries. After stabilizing, he spoke with law enforcement, providing his version of the events that led to the tragedy. Allegheny County Police, in consultation with the District Attorney’s Office, moved swiftly. Nimajuan-Sitan was charged with criminal homicide and taken to Allegheny County Jail, where he awaits his preliminary arraignment. The weight of the charge hangs heavy, a stark reminder of how quickly a moment of conflict can spiral into irreversible loss.
As detectives continue to piece together the events of that fateful night, authorities have assured the public that there is no ongoing threat. “All parties involved are accounted for,” police stated, emphasizing their commitment to a thorough investigation. The Allegheny County Police Homicide Unit is working diligently, collaborating with the District Attorney’s Office to determine the full scope of the incident and whether additional charges may be warranted.
The community of West Mifflin, known for its tight-knit neighborhoods, now grapples with the shock of this violent episode. Thompson Run Road, typically a quiet stretch, has become the backdrop for a tragedy that underscores the fragility of life and the devastating consequences of a single moment of rage. As the investigation unfolds, authorities are urging anyone with information to come forward, offering anonymity through the Allegheny County Police Tip Line at 1-833-ALL-TIPS.
For now, the home on Thompson Run Road stands silent, its walls holding the secrets of a night that ended in bloodshed. The loss of Mario Alejandro Perez Santos and the charges against Andy-Steven Nimajuan-Sitan serve as a haunting reminder of how quickly a day of laughter can turn into a night of sorrow.