It can happen in an instant.

One moment you’re sitting on BART, quietly reading the Mercury News on your phone, and the next, someone has snatched the phone out of your hand and is running out of the train before you can stand up or say a word in protest.

Though three-quarters of “cellphone snatching” cases don’t involve force or threats of violence, a quarter of them do, BART officials said Tuesday, as part of an awareness campaign to caution riders to keep their electronics close. Cellphone and electronic thefts have more than doubled in the first four months of the year, according to BART. And, they can happen quick, as a surveillance video BART provided shows.

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In the 20-second video, the time it takes for a thief to steal a passenger’s phone happens in fewer than three seconds.

“Many of the cellphone thefts on BART are crimes of opportunity,” Interim BART Police Chief Ed Alvarez said in a statement Tuesday. “Thieves will board a train and wait until the moment it arrives at a station and the doors open to snatch a phone and make a quick escape.”

From January to April of this year, there were 402 incidents of cellphones or electronics stolen out of riders’ hands, more than double the 191 incidents reported during the same time period last year. Of those, roughly a quarter of the incidents this year involved the use of force or the threat of force, BART said, compared to roughly 30 percent of the incidents during the same time period last year.

The BART Police Department — which faced increased scrutiny last year following the brutal stabbing death of 18-year-old Nia Wilson, along with increased violent crime on the system — has hired 50 police officers since the beginning of 2018, said BART spokesman Chris Filippi. But it still has 22 vacant positions it’s seeking to fill.

In the meantime, BART is urging its passengers to be extra cautious when the train is approaching a station or when they are waiting on a station platform for the train to arrive.

Here’s what you can do to stay safe: