Bomb threats were called into three D.C. schools on Friday, forcing evacuations and heavy police responses for the fourth consecutive day in the city.

       Police said nothing hazardous was found at the schools: Calvin Coolidge High in Northwest Washington, and the upper and lower campuses of Digital Pioneers Academy in separate parts of Southeast.

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       Authorities arrested two 16-year-old boys in connection with some of the threats made to eight D.C. high schools on Wednesday. Both were charged as juveniles with making terrorist threats.

       On Friday, City Administrator Kevin Donahue said that the two youths were not connected to each other and that police do not believe they intended to carry out their threats.

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       “Historically, in other parts of the country, it was a copycat pattern that emerges with school bomb threats,” Donahue said on a weekly call of administration executives and members of the D.C. Council. “That’s what we appear to be seeing so far.”

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       Regardless of the intent, Donahue said, “These are criminal acts. They are not harmless pranks. They will be investigated and taken very seriously.”

       Additional threats were made to schools in the District on Thursday. Police have said the threats were largely similar — all made through telephone calls, and most threatening a bomb and warning that people inside had limited time to leave.

       One caller on Thursday told a D.C. school staff member, “I’m gonna blow the school up in fifteen seconds,” according to a police report. A call to another school around the same time gave a 30 second warning.

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       Most of the callers are male, police said, though one female threatened that she had a gun and a bomb in the building, the police report says.

       Authorities said nothing hazardous was found at any of the locations. Someone threatened to “shoot up” a high school in Arlington, Va., on Thursday. That same day, four high schools in Prince George’s County, Md. were targeted, and police said a high school and a middle school in that county received bomb threats on Friday.

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       The first in this series of bomb threats gained widespread attention because it occurred as second gentleman Doug Emhoff, the husband of Vice President Harris, was visiting Dunbar High School in the District on Tuesday.

       D.C. police said they have not linked either of the two teenagers arrested this week to that case. The FBI, the U.S. Secret Service and the Bureau of Alcohol, Tobacco, Firearms and Explosives are assisting in the investigation.

       Katie Mettler contributed to this report.