Prince George’s County police charged two men Monday in unrelated slayings in which robbery was the motive and the victims were described as among the most vulnerable — a 71-year-old woman who used a walker and a student on his way to school.

The suspect in the student’s slaying turned 20 Monday and has been in custody since October, held in connection with what police described as an armed robbery spree in Capitol Heights. The man charged with killing the elderly woman had helped the victim with odd jobs and stabbed her while stealing her flat-screen television, police said.

The two killings came amid a spate of deadly violence in recent months that included the slayings of five other high school students. In the latest killing, a man was gunned down in Lanham Sunday morning in what police said was a gang-related shooting.

On Monday, authorities hailed the charges in the two cases as pushback against crime and a step toward restoring “a sense of relief to the community.”

“Our goal is to find answers, and today we have that,” Prince George’s County Police Chief Mark A. Magaw said at a news conference. Noting the ages of the victims, the county’s state’s attorney, Angela D. Alsobrooks, said that “we will do everything in our power to prosecute those who target the most vulnerable — the old and the young.”

Travon Donnell Bennett (Prince George's County Police)

On Monday evening, nearly 300 people, including Magaw and Alsobrooks, packed the gymnasium of the Suitland Community Center to express frustration over the killings.

“These violent acts . . . really have traumatized our community and our police department,” Magaw said. “It has to stop.”

County police have now made nine arrests in four of the student slayings, including one in which they say a 15-year-old was shot over a pair of Timberland boots he had bought for his girlfriend.

One of the cases in which charges were brought Monday was the Sept. 11 shooting of 18-year-old Marckel Ross, who ran track at Central High School in Capitol Heights and modeled and was considering the military or college after graduation.

Police said the suspect, Travon Donnell Bennett, 20, of Bryans Road, a town in Charles County, did not know Ross. They think Bennett set out to rob the student, who was walking alone from his home to Central. Police have been unable to determine whether anything was taken from the teenager.

Charges against Bennett include first-degree murder, assault and armed robbery.

His public defender did not return calls seeking comment.

Six Prince George’s County students have been killed during the 2012-13 school year as a result of violence.

Elizabeth Ross, mother of the victim, said a homicide detective visited her over the weekend to tell her charges were imminent. “I just hope he never is able to come back to the streets,” Ross said of Bennett. “Marckel just happened to run into the devil that day, waiting for someone to rob.”

The teenager’s father, Norman Thompson, said an arrest alone wouldn’t bring solace to the family. “We’ll see what happens in court,” he said when reached by phone. “We won’t have closure until he’s tried and convicted.”

Maj. Michael Straughan, head of the Prince George’s County Police’s criminal investigation division, said detectives got a break in the case on Oct. 31 when Bennett was among three suspects arrested in an armed robbery in Capitol Heights in which a man’s cigarettes were taken. Straughan said patrol officers recovered a .38-caliber revolver, and ballistic tests linked it to the shooting of Ross.

Police said they also linked Bennett and others to a series of armed robberies, including one in May in which a man was shot.

Police also said they arrested a suspect in the Feb. 9 killing of 71-year-old Geraldine McIntyre in her Capitol Heights home. Mc­Intyre, known as “Miss Geraldine,” used a walker or wheelchair to get around.

James Alphonso Ward, 46, of Capitol Heights has been charged with first- and second-degree murder and assault. Police said Ward has been jailed on burglary charges in an unrelated case.

Straughan said detectives got a break in this case Friday when someone who lived on McIntyre’s street told detectives that on the day McIntyre was killed, he saw a man he knew as “Juju” walking toward her home. He said that a short time later, the man came to his door with a 32-inch flat-screen television and sold it to him for $40, according to court documents filed Monday.

The man later learned that a TV had been taken from Mc­Intyre’s house, the documents state. Straughan said Ward knew McIntyre and at times helped her with odd jobs and preparing her food.

Neighbors said at the time that the victim lived with her adult daughter and that the daughter had been hospitalized before her mother was found dead. Both had limited mobility.

Neighbors also said that in the summer McIntyre would sometimes leave the door of her small yellow house open so people delivering meals could get in.

Keith L. Alexander and Matt Zapotosky contributed to this report.