Two bomb threats called in to Westfield High School during the first week of school were hoaxes known as “swatting,” in which a caller makes a false report to police to draw an overwhelming response, according to Fairfax County police and schools officials.

The first call came in after school had let out Tuesday, the first day of the school year, leading to a lockdown that affected more than 200 students and staff members. A second fake threat was called in to Westfield on Thursday.

The false alarms at Westfield, in Chantilly, were the only major interruptions during an otherwise smooth opening week of the school year in the Fairfax school district, which expects to enroll 186,785 students this fall.

“Swatting” pranks in the United States have become more frequent in recent years, often originating from teenagers with the goal of prompting police to deploy special weapons and tactics, or SWAT, teams to specific locations. The aim is to cause disturbances or to rattle particular individuals. According to a report from the Northern Virginia Regional Intelligence Center (NVRIC), swatting calls often come from “anti-social, easily led or troubled teens (young adults) seeking social acceptance.”

Authorities say such calls can be dangerous for police officers and those in the targeted buildings. Police often go to the scenes with guns drawn and on high alert because they do not know whether they are responding to false reports or to something far more serious, such as the massacres at Virginia Tech in 2007 and at Sandy Hook Elementary School in Newtown, Conn., in 2012. The false alarms also can cost police departments tens of thousands of dollars.

The NVRIC report notes four recent swatting incidents in Northern Virginia, including one call placed in July reporting that armed men wearing tactical clothing and masks had entered a bank. In December, a caller claimed that people inside a government employee’s residence were being raped and murdered. The document noted that the caller had intimate details about the family, including family members’ names.

According to internal school security documents obtained by The Washington Post, the hoax at Westfield on Tuesday was the first known case of “swatting” at a Fairfax County school. About 3 p.m., the school received an anonymous Skype call from someone claiming to be in the building with a rifle and explosives. The caller demanded money, according to the school documents.

Administrators quickly locked down the school while police officers swept the building and surrounding campus with two dog teams. Police eventually determined that the school was not in danger.

“The caller spoke to school personnel, made unspecific threats regarding the building and weapons,” said Officer Lucy Caldwell, spokeswoman for Fairfax County police.

Schools spokesman John Torre said Westfield received a similar call Thursday. The school did not go into lockdown, but administrators in the building “took precautionary steps” to keep students safe, he said.

Caldwell said police received a report of a bomb threat at Westfield at 1:56 p.m. Thursday after an anonymous caller made threats against the school. Caldwell said that extra police officers were sent to the school to monitor dismissal but that no other action was taken. Both cases are under investigation.

On Aug. 27, Loudoun County High School in Leesburg was placed on lockdown about noon while police searched the premises. The episode was later determined to be a swatting hoax. On Wednesday, police officers swarmed the University of Maryland campus after a caller claimed that an assailant had taken hostages. The report was false.

Prank calls placed via “voice over Internet protocol” phone services such as Skype can be difficult for police to trace. In Virginia, anyone caught calling in a threat “to commit harm on school property” can be charged with a felony punishable with a prison sentence of up to five years.

Justin Jouvenal contributed to this report.