University officials will formally reactivate the Yale Police Department’s undercover street crime unit Tuesday in response to rising concerns about on-campus crime, Yale Police Captain James Perrotti said.

Two plainclothes officers will patrol campus at night in an unmarked car, Perrotti said, especially in areas where students have said they feel unsafe. The street crime unit has been used on and off in the past, Highsmith said, but has not been activated in several years.

“We haven’t needed one in a while,” Highsmith said.

The decision was made following a robbery of two students on Old Campus Dec. 18, Deputy University Secretary Martha Highsmith said.

At about 11 p.m., Dec. 18 two female students were accosted by a stocky black man in his mid to late 20s in front of Vanderbilt Hall, University Police Lt. Michael Patten said in an e-mail. The man pointed a black object at the women, but it is unclear if the object was actually a weapon, he said.

Patten said the man took the purse of one of the students and left on foot. There have been no arrests in the case, which police are treating as an armed robbery, but police have several suspects, Perrotti said.

Armed robbery tends to increase in the winter months, Perrotti said, because criminals can better conceal a weapon in long clothing. Crime in New Haven has fallen dramatically in the last decade, but Perrotti said recent evidence suggests an end to the decline.

“We see the things in New Haven,” he said. “There’s certainly been a leveling off, there may have even seen a slight increase [in robberies]. There’s certainly more commercial robberies going on. It’s a climate where the decision to activate the street crime unit seems appropriate.”

However, Highsmith said the most recently compiled Yale crime statistics indicate that campus crime against persons, the crime the undercover unit is designed to prevent, remained stable in 2003.

The robbery Dec. 18 comes on the heels of several other armed robberies of students that have occurred this year. A woman was robbed and fondled in her off-campus apartment by a man with a gun Dec. 14. In September, Perrotti sent an e-mail to students and faculty informing them that what police believe to be a single suspect, mugged two students at knifepoint on campus.

Though the unit will not be officially reactivated until Tuesday, plainclothes officers began patrolling campus immediately after the Dec. 18 robbery. Perrotti said the undercover police had already showed their usefulness in the few days they were in operation last semester.

“They made some arrests,” he said. “They also were able to make a lot of evaluations of who’s on the street, who’s in the neighborhood.”

The unit will likely be in operation until the end of the semester, Perrotti said. At that point, he said, it will be reevaluated to determine if it is still needed.

Perrotti said there was debate in the department about whether the undercover unit’s existence should be made public.

“It’s good for the campus community to know what’s out there,” Perrotti said. “If the bad guys find out, good.”