Abigail Mentzer was riding the New York City subway to a doctor’s appointment when she says an AirDrop request popped up on her iPhone. A preview image of the file showed a CD with the handwritten message, “Songs I’ll choke you out to while wrecking your uterus.”

Feeling disgusted and threatened, she looked around the train car, wondering who had sent it. Then, three more messages came through, including images of a woman’s bare behind and more offensive language.

“I had never gotten an AirDrop from a random person before. I didn’t know this could happen,” Mentzer, 35, an actress and dancer, told NBC News recently. “I guess it makes sense in this day and age, to find new, inventive ways to harass people.”

AirDrop is an iPhone feature that allows users to share photos and other files quickly, without disclosing their phone number or email address. The message arrives from the name of the iPhone (Mentzer’s message came from “iPhone 9”), giving the sender anonymity. Users need to be within roughly 30 feet of each other to send an AirDrop, which operates over Bluetooth and Wi-Fi.

Crowded spaces are fertile ground for AirDrop harassers. Public transport in particular seems to be a danger zone. The New York subway system has been the scene of several disturbing incidents, and a reporter for HuffPost UK reportedly received over 100 lewd AirDrops while riding the London Tube.

The British Transport Police, who monitor the nation's railways and the London Underground, said they receive two to three reports about "cyberflashing" via AirDrop a month.

“As with other forms of sexual harassment, we believe that cases of cyberflashing, which can involve the sending of unwanted, threatening or explicit sexual communications, goes largely unreported — either because victims don’t feel the incident is serious enough to report or simply because they don’t know where to turn,” Detective Inspector Ashley Cooper said in an email.

Saving an offensive AirDrop image can help the police with their investigation, said Nikki Nagler, a spokeswoman for the British Transport Police. If a suspect is identified and arrested, images on his phone can be compared with the offensive image on the victim's phone. "As you can imagine, identifying suspects with sexually explicit photos can be challenging," she said in an email.

She added that it is "completely understandable" that victims will not want to store unwanted sexual images on their phones, and can provide police with a copy in that case.

Default settings on the iPhone are set to accept AirDrops from contacts only. But because the purpose of AirDrop is to quickly share files without exchanging contact information, many iPhone owners change the setting to “everyone.”

Navigating the settings can be confusing, and there’s no warning on the iPhone about the implications of setting AirDrop to “everyone.”

“I’m not great with technology. Someone had to show me how to change AirDrop from contacts to everyone,” said Mentzer, who uses AirDrop to exchange footage of herself practicing for auditions with colleagues at various studios in New York.

The file from the sender can be rejected, but the preview image is visible regardless of whether the message is accepted.

A spokeswoman for Apple declined to comment on the phenomenon of viewing unwanted messages. She said users can keep the settings on “contacts only,” or turn off Bluetooth and Wi-Fi.

An AirDrop sent to Abigail Mentzer from an anonymous source while she rode the subway in New York City in October. Courtesy Abigail Mentzer

Oumou Fofana, 24, a student from Brooklyn, also says she received unwanted AirDrops on the New York subway. She was riding the No. 3 train home from school and scrolling Instagram when a message request appeared on her screen. It was a picture of a penis, followed by an image with the message, “Send something back like this if you’re not scared.” The phone that sent it was identified only by an emoji — a flexed bicep.

“I was disgusted,” Fofana told NBC News. “I felt unsafe. I had to ride 20 more minutes to get home. I sat there wondering if the person was going to follow me when I got off the train.”

The sender is anonymous, but so is the receiver: Unless a sender knows the name of the receiver’s iPhone, there’s no way to be sure who the AirDrop is going to.

“I looked around to see if I was the only one, but I couldn’t tell,” Fofana said. “It makes sense because I tried to act like I didn’t see it, in case he was watching me.”

Anonymous AirDrops are easy to get away with. The New York Police Department said in an email that it does not keep track of AirDrop abuses, but that if they were reported, they would be investigated under harassment claims.

The anonymity of an offensive AirDrop, combined with the knowledge that the offender is nearby, makes it a threat. For survivors of sexual assault, it can also cause acute emotional distress.

“It has all the markers of trauma: unwanted, unpredictable and out of control,” said Helen Wilson, a clinical psychologist at Stanford University who works with students overcoming sexual violence. “For someone with past sexual trauma, it could cause stress and a physiological response.”

Mentzer is a survivor of sexual assault, and said receiving the offensive AirDrop was triggering.

“I was terrified of men for a while because of something that happened when I was a young teen. I’d get panic attacks if I was alone with men,” she said. “I’m OK now, but the images gave me that fear for a moment. Is this guy gonna follow me? Does he know who I am?”

Wilson said that receiving an image like Mentzer’s could cause physical responses to stress, like shortness of breath or tightness in the chest.

“Trauma can make the world seem more dangerous and scary to survivors,” Wilson said. “They can have reactions that seem out of proportion to others.”

She said it’s helpful to recognize the stress. “Acknowledge that it’s a scary experience, a real stress. Talk about it with people you trust and find activities that make you feel calm and in control again, like meditating or exercise.”

Mentzer said her way of dealing with it was posting it on social media for everyone to see. The support she received from friends provided some comfort, but she said that, as a woman, she’s had to get used to events like this.

“I was taken aback at first,” she said, “but it’s just another day in the life of #MeToo.”