Somerset authorities will only say the second teen is a 17-year-old senior at Immaculata High School in Somerville.
The second arrest came on Friday night after police discovered several firearms and shotgun shells that matched the shells reportedly found at the 16-year-old's home. Police also found bullets, black powder and explosive devices at the home of a 16-year-old high school junior. Police seized them as well as the teen's computer.
The first teen was arrested Thursday after a police resource officer at the school was told about threatening comments the teen made.
"I'm glad the school was able to find out and someone was able to speak up," one parent said.
Officials say the 17-year-old is a competitive rifle shooter and the weapons found in his home were registered to his father.
After further investigation Somerset officials determined the 17-year-old provided the 16-year-old with the shotgun shells to make the homemade explosives. Officials also say, the two teens purchased PVC piping, wiring, light sockets and light bulbs from a Home Depot in Bridgewater -- and began constructing the explosives.
Prosecutor Wayne Forrest said the 17-year-old has been charged with third degree possession of destructive devices.
Forrest said the "Columbine"-type attack was planned for December 14 at Bridgewater-Raritan High School.
The school was closed Friday as authorities searched the buildings. No explosives were found.
Bridgewater Superintendent James Riccobono credited the vigilant action of another student for making authorities aware of the threat.
"For years, we've promoted the hope that students who have knowledge of any possible threat to the well-being of the school report that information to a school official. In this instance, this hope was materialized with a report from a member of the student body," Riccobono said.
The 16-year-old is being held at the Middlesex County Juvenile Detention Facility on numerous charges, including attempted murder and possession of explosives.
Authorities plan to search Immaculata High School prior to students returning on Monday.
Neither teen's name has been released.
(Information from WABC-TV and Associated Press).