Two groups of tourists were robbed at gunpoint on the National Mall, just hours after the police chief declared a crime emergency in the city in response to a string of violence that included the killing of a British activist.

The activist, Alan Senitt, was attacked in the Georgetown area on Sunday, his throat was slit and police say the attackers attempted to rape his companion. It was the 13th homicide in the city this month. Robberies are up 14 percent, and armed assaults have jumped 18 percent in the past 30 days.

On Wednesday, U.S. Park Police were looking for connections between the latest Mall robberies and three similar incidents in the area in late May. There have been no arrests in any of those cases.

On Tuesday night, two women from Texas were robbed at gunpoint by two men dressed in all black, said U.S. Park Police Sgt. Scott Fear. About 15 minutes later, a family of four from Missouri was robbed by suspects with the same description, he said.

Though no one was injured, Fear said there were similarities to three violent attacks on the National Mall in May. In one case, a 17-year- old woman was sexually assaulted.

"We try to prevent this from happening," Fear said. "We're going to reallocate our resources. We're going to see what improvements we can make."

District of Columbia Police Chief Charles Ramsey declared a crime emergency in the city after Senitt, a volunteer for the potential presidential campaign of former Virginia Gov. Mark R. Warner, was killed.

The crime emergency declaration allows commanders more flexibility to adjust officers' schedules and reassign them to high-crime areas.

The tourist-friendly National Mall, which is under the jurisdiction of Park Police rather than D.C. police, is usually considered safe. But the recent crimes against tourists have raised calls for a larger police presence. Police are asking Mall visitors to "be our eyes and ears," Fear said. "We're going to ask them to be vigilant."