The two Drexel students were watching television on the second floor of their Baring Street home in West Philadelphia when two men materialized, handkerchiefs over their faces, one aiming a handgun at them.

The intruders bound the 22-year-old students' wrists with plastic ties. The gunman hit one in the back of the head with the weapon, demanding, "Where's the money?" and the two ransacked the house, took $800, and were gone.

One departed with a warning: "I shot two cops in Baltimore, and I'm not afraid to shoot you."

It didn't take long for police to guess why the Powelton Village building was targeted in May. In common rooms, officers found a digital scale and items associated with marijuana growing - items the six roommates claimed they knew nothing about.

Increasingly, police say, a violent home invasion is the price some pot-dealing university students pay for taking up a dangerous side job.

Students who deal drugs make up a tiny minority of Drexel and Temple students, and home invasions near those campuses are rare.

But when it happens, police say, it's almost always when the target has drawn the attention of neighborhood drug dealers by selling marijuana.

Since November, there have been at least seven home invasions around the Temple campus in which police believe drugs were a factor. There have been at least four such robberies of Drexel students since April 2010. Police could not recall one home invasion involving students that was not drug-related.

"These kids are cutting into the trade of the people who live in this neighborhood," said Philadelphia Lt. John Walker of the Southwest Detective Division. "To them, it's a few hundred dollars, but to someone else, that's their livelihood. . . . The students don't realize how they're affecting the economy."

The home invasion is the professional dealer's way of sending a message to the amateur: Knock it off.

"It puts the ultimate level of fear into them," Walker said.

To a local dealer, robbing a college student - the lowest rung on the drug-dealing ladder - is low-risk, high-reward. The student is likely to be unarmed and to offer little resistance but plenty of cash, laptops, and more. Students often get rid of drug evidence before calling police, a delay that can make it harder for police to catch the assailants.

"The risk isn't the same as robbing a bona fide drug dealer," said Philadelphia Capt. Branville Bard of the 22d Police District, which covers the North Philadelphia neighborhood around Temple. "Your typical drug organization is armed to the teeth. These kids are caught off-guard."

Students targeted are usually assaulted, mostly pistol-whipped or tied up. The Inquirer is not identifying students who have been robbed unless they were arrested.

"My main concern is that one of these kids is going to get hurt or killed," Bard said. "Until now, everyone's been compliant. But it's inevitable that one of these kids will try to defend themselves."

Even when someone is compliant, Bard said, "they can still get shot."

The gunman who approached the house in the 3500 block of Spring Garden Street on the morning of Sept. 2 knew there was a stash of pot inside, police said. Getting inside was as simple as ringing the bell and asking to buy some pot.

After a student let him in, the man drew a gun and forced the student into another room. There, the intruder found five other students.

Emboldened by the six-on-one ratio, one student knocked the gun out his hand, and the intruder fled.

As usual, Walker said, the victims were lucky. The student who knocked away the gun might have missed, or the gunman could have grabbed it from the floor.

"Then we get someone opening fire in a room," Walker said. "We could have had six people shot."

Students who start selling drugs can be dangerously naive, Bard said.

To acquire a supply, some seek out dealers who operate just blocks from their homes. Buying a few hundred dollars' worth of marijuana is enough to draw attention; students never seem to consider that a dealer may follow them home. Some students have pot delivered to their doors.

"Kids are meeting these guys right where they lay their heads," Bard said. "These dealers realize they cannot only get the money, but they can get that product back and resell it."

In every way, Bard said, students underestimate who they're dealing with.

"They don't realize that the folks they're associating with are lifelong criminals who've done far worse than sell marijuana," Bard said. "It's painfully obvious that they don't see the targets they're putting on their backs."

Police reports show that the invasions near Temple occurred suddenly and were over in minutes.

One night in November, four roommates in the 2100 block of North Carlisle Street had just ordered a pizza when two men forced their way in the door at gunpoint. They made the roommates lie on the floor and pistol-whipped two of them.

In February, several men kicked in the door of a house in the 1900 block of West Norris Street, tied T-shirts over the eyes of two students, pistol-whipped the pair, and forced them to kneel on the floor. "We've been watching the house," one assailant said. "Where's the money at?"

After police arrived, one victim admitted that he had been selling "a little bit" of marijuana for a few months, and that he occasionally let buyers into the house.

Though police link most invasions to drugs, student arrests are rare. By the time police arrive, most evidence has been stolen or destroyed.

Even if a student admits to selling drugs, Bard said, "There's a slim chance of bringing charges, and an even slimmer chance of a successful prosecution."

Officials from Temple and Drexel are aware of the home invasions. A representative from Drexel said the university has a zero-tolerance policy for criminal activity.

"When Drexel is made aware of any students participating in illegal behavior, the university places them on indefinite suspension pending the outcome of the investigation," said Niki Gianakaris, Drexel's director of media relations. Temple declined to comment.

In each house, Walker said, there is often at least one roommate who was not aware of the dealing, and sometimes those students report the robberies. Other times, the students who sold drugs are scared enough to call 911.

"We lecture the kids after it's over," Walker said. "We tell them, you don't need to admit to selling weed, but just know that you're not being picked on at random."

According to court records, one former Drexel student who came close to being shot in an invasion may have taken the experience to heart.

Kevin Ulrich, 23, was with his roommate in their apartment in the 4000 block of Ludlow Street in University City in April 2010 when two men burst in. The roommate was handcuffed, and one man hit Ulrich in the head with a gun, asking, "Where is it?"

As the man struck Ulrich, Walker said, the gun accidentally went off, firing into a wall. The intruders fled and neighbors called police.

Officers found $27,000 in cash, along with 2,700 grams of marijuana stashed in a Drexel lacrosse duffel bag - an estimated street value of $55,000, Walker said.

Ulrich, whose lawyer did not return calls for comment, was charged with possession with intent to deliver. He was convicted of possession in December, and given probation.

He told the court he planned to join the Marines.

Contact staff writer Allison Steele at 215-854-2641 or asteele@phillynews.com.