A bank was held up Thursday in Loudoun County in what apparently was the 14th bank robbery or attempted bank robbery in the Washington area in 14 days.

The holdup in Sterling on Thursday appeared to be the area’s first of 2014, and it came two days after three men were arrested in a New Year’s Eve holdup in Arlington County.

In new information about Tuesday’s Arlington incident, the U.S. attorney’s office for Virginia’s Eastern District said about $47,000 in cash was taken from teller drawers after a robber jumped a counter at a Wells Fargo branch..

The amount was several times the average of about $7,600 taken in the 5,014 robberies of financial institutions reported to the FBI in 2011. (The figure includes armored trucks.) From available data, it appeared to be one of the larger hauls in a recent Washington area robbery.

Thursday’s robbery in Loudoun occurred shortly after 9 a.m. at a PNC Bank branch in the 100 block of Free Court, the county sheriff’s office said. Authorities said a man wearing a black toboggan hat entered the branch shortly after 9 a.m. and demanded cash. He was still being sought Friday.

The string of robberies began Dec. 20 in the Hayfield area of Fairfax County. Another occurred the day after Christmas, but was first reported Thursday by Montgomery County police. They said it occurred shortly after 4:30 p.m. at a M&T branch in the 7900 block of Norfolk Avenue in Bethesda.

Police said the robber approached a teller, produced a note and said he was armed. He fled with an undisclosed amount, police said. Detectives think the same man held up a bank in Potomac on Dec. 21, police said. Police said the robber in the more recent incident wore the same baseball hat seen in surveillance photos from the Potomac robbery: a dark hat with an “S”-shaped emblem.

Fairfax police have said they thought that two of the robberies there since Dec. 20 were linked.

In court papers, an FBI agent said the three men arrested by the FBI and Arlington police Tuesday had been linked by an informant to four other bank robberies or robbery attempts. None of the four, however, was part of the recent series.

The 14-day period in which the 14 robberies occurred included two Sundays, on which few banks are open, as well as Christmas and New Year’s Day.