Two violent incidents kept Bostonians on edge over night, just days after the bombing of the iconic Boston Marathon.
It started with the shooting death of a college police officer Thursday night and ended with a carjacking that may have involved explosives early Friday.
Police believe at least one suspect used explosives against their officers in the second incident.
Howling sirens pierced the night quiet, and flashing lights lit up the darkness, as dozens of officers from state and city police responded to the deadly shooting on the campus of the Massachusetts Institute of Technology in Cambridge. Hardly an hour later, they were chasing at least one suspect in a hijacked vehicle less than five miles away.
Police cornered the vehicle in the suburb of Watertown and arrested two suspects. They may have used explosives in their futile attempt to elude police, police spokesman Dave Procopio told CNN.
CNN photographer Gabe Ramirez arrived in Watertown as the chase ended.
Two suspects, one stripped down
"Police were in a standoff with the vehicle just down the hill," Ramirez said. They ordered one suspect out and commanded him to strip down completely naked before putting him in a patrol car, which did not leave the scene.
FBI agents approached the squad car, and police ordered the suspect back out of the car. FBI agents questioned him -- still fully undressed -- on the sidewalk.
Later, police lead the man in handcuffs to a patrol car.
Officers quickly locked down the streets of the Watertown neighborhood after isolating the vehicle. The Boston Police Department said in a statement that they consider the incident to be still active and ask that residents remain in their homes.
Police carrying assault rifles ran down the streets, according to CNN affiliate WCVB, which broadcast images from the area.
In an early phase of the lock down, a man could be seen lying face down on the street in a surrender pose with his hands outstretched in front of him and his legs crossed. It is unclear if he was one of the suspect arrested.
Police requested that people in the area turn off their cell phones.
CNN Senior producer David Fitzpatrick reported hearing a loud bang Friday morning in Watertown after the police had stopped the vehicle, but police have not confirmed an explosion.
Dozens of police from various units arrived in Watertown, some in SWAT uniforms, others wearing helmets. Large crowds gathered around a trove of emergency vehicles that had congregated in the neighborhood, WCVB reported.
Homeland Security Investigations has deployed agents to Watertown to try to determine if there is a connection between the Marathon bombing Monday and tonight's violence, a Homeland Security spokesman told CNN.
Police also said they were going door to door, street by street, searching the Watertown area.
It is still unclear if the arrest in Watertown was related to the shooting on the MIT campus or any other incident in the Boston area.
Residents in the area have been on edge after two bombs ripped through the crowd near the finish line of the Boston Marathon, killing three people and injuring 178 others.
Federal, state and local agencies are still investigating the marathon bombing, and police were seeking two suspects in the attack, who were still at large.
How the night erupted
The mayhem began, after a university police officer died in a shooting on the Massachusetts Institute of Technology campus in Cambridge late Thursday, state police spokesman Lt. Mark Riley said.
The MIT officer was responding to a disturbance when he was fired upon, according to the state district attorney's office. He sustained "multiple gunshot wounds."
State police and the FBI were called in after the shooting and found the campus policeman near Building 32 on MIT's campus. He was taken to Massachusetts General Hospital, where he was pronounced dead, the district attorney's office said.
The university issued a statement of condolence. "MIT is heartbroken by the news that an MIT Police officer was shot and killed in the line of duty," it read.
Dozens of officers surrounded and cordoned off the building, known as the Stata Center, which houses computer science laboratories as well as the department of linguistics and philosophy, according to MIT's website.
The university, which lies adjacent to the city of Boston, had requested people stay away from the building, but have since said the campus is safe and that the suspect is no longer present.
Students and faculty received e-mails to alert them to the event, CNN affiliate WCVB reported.
Many news media outlets are in the area covering the investigation into Monday's bombings and arrived at MIT to cover the shooting, WCVB reported.
Within an hour, police were called to respond to the carjacking, which lead them on the chase through Watertown.