Two Pennsylvania teens arrested last week are being charged as adults after authorities say the pair were allegedly "in the planning stages of a school shooting."

The charges come after the Upper Yoder Township Police Department investigated a trespassing report made by a concerned parent regarding two teens, ages 16 and 17, at Westmont Hilltop High School in Johnstown on Wednesday, the Cambria County District Attorney’s Office said Saturday in a news release.

OXFORD SCHOOL OFFICIALS ACCUSED OF DESTROYING SCHOOL SHOOTING EVIDENCE, JUDGE ORDERS THEM TO PRESERVE IT

The 16-year-old had allegedly allowed the 17-year-old, a former student who was barred from the school by a court order, into the school building, the Meadville Tribune reported.

Westmont Hilltop School District Superintendent Thomas Mitchell said in a letter to parents on Thursday that there were no issues during the breach.

"Our students and employees were not in danger, but the recent violent acts in Michigan remind us that we all need to remain vigilant to keep one another safe," the letter said.

The investigation into the trespassing led authorities to execute multiple search warrants in the 100 block of Forest Edge Drive, the district attorney’s office said.

Authorities seized five weapons from the home of the 16-year-old, including one pump-action shotgun and four AR-15 style semi-automatic rifles, the paper reported. Police said the teen had access to at least two of those guns at different times.

While authorities alleged the teens were in the planning phase of a school shooting, no motive was immediately given, the report said.

The 16-year-old was charged with the first-degree felonies of conspiracy to commit terrorism and conspiracy to commit aggravated assault, third-degree felony of conspiracy to commit criminal trespass and possession of a firearm by a minor, a first-degree misdemeanor.

The 17-year-old faces charges of conspiracy to commit terrorism and conspiracy to commit aggravated assault, both first-degree felonies. He was also charged with an attempt to commit aggravated assault.

CLICK HERE TO GET THE FOX NEWS APP

Both teens were taken into custody without incident and were being held at the Cambria County Prison ahead of their preliminary arraignments.

The arrests come just over a week after a 15-year-old allegedly opened fire at his high school in Oxford, Michigan, killing four fellow students and wounding six others, including a teacher. He has been charged as an adult with murder, terrorism, and other crimes in the attack. 

Since the shooting in Michigan, students in New Jersey and South Carolina have been arrested after allegedly making threats against their schools.