D.C. Police Chief Cathy L. Lanier. (Marvin Joseph/The Washington Post)

D.C. police in the past week arrested 13 suspects in 19 robberies of people and businesses across the District, including one man who authorities said was charged in six holdups in which a gun was used or hinted at.

Police Chief Cathy L. Lanier highlighted the arrests at a news conference Friday to demonstrate the department’s aggressive stance against a crime that is up 30 percent in the District in the first month of 2016.

The announcement came the day after a female student on her way to a class at Trinity Washington University was beaten and robbed of her cellphone while walking on Michigan Avenue along the edge of the Northeast Washington campus. The school said police were looking for eight female teenagers wearing what appeared to be high school uniforms.

Ann Pauley, a spokeswoman for the small private university that enrolls students mostly from the District, said the victim remained hospitalized Friday. No arrests had been made.

“It’s sobering for this to happen right on the edge of our campus,” Pauley said. “We are stepping up our own security. We are in constant touch with the police. There definitely has been an increase in crime since last spring or summer, and it does not seem to have abated.”

D.C. police last month set up a robbery task force involving other agencies, including the Metro Transit Police, and a prosecutor with the aim of quickly identifying patterns and individuals committing multiple offenses. Officials said the FBI helped with some of the recent cases.

“We have a small number of people who commit an awful lot of crime,” Lanier told reporters Friday. In addition to the man charged in six cases, another man was charged in three robberies and another in two.

“A very small number of suspects and a very large number of crimes,” Lanier said, noting that the recent arrests were for robberies in all corners of the District. The crimes, she said, have had a “huge impact across the city.”

Lanier singled out Michael Williams, 54, of Northeast, who was charged with six counts of robbery in connection with attacks on stores from Jan. 12 to Jan. 31.

The chief said Williams was arrested on a warrant Friday morning, days after surveillance video from one of the robberies was made public.

“He did not appear to make a lot of effort to conceal his identity,” Lanier said.

In some cases, police said, Williams threatened cashiers or workers with a demand note or flashed a gun. In other cases, victims reported seeing bulges in the robber’s pocket that resembled a weapon. Police said Williams robbed two stores hours apart on Jan. 12, one on Jan. 26 and three on Jan. 31, all in Northeast Washington.

The three robberies on the last day of January occurred at 1:45 p.m., 2:05 p.m. and 10:20 p.m. In the three cases, police said, the robber got money when he threatened the clerks, compelling them to open their cash registers.

Other cases in which suspects were recently apprehended include robberies targeting pedestrians and cellphone stores, Lanier said. Police said an arrest also was made after a man forced his way into the attendant’s booth at a parking garage on 13th Street NW on Feb. 1, put a gun to the attendant’s head and took money.

The robbery Thursday on the Trinity campus prompted university President Patricia McGuire to issue a campus-wide crime alert.

The school, with about 2,000 students, is just south of Catholic University and near Edgewood and Brookland. Pauley, the spokeswoman, said the student suffered multiple cuts, bruises and scratches, and although she initially was treated at the scene, she later checked into a hospital. Pauley said the victim lives off campus and had been walking to an evening class.