The FBI has opened civil rights investigations in seven cases in which Prince George’s County police shot or allegedly beat people, bringing to more than 30 the number of federal probes involving county officers in the past two years. An FBI spokesman said yesterday that four of the investigations will seek to determine whether police violated the civil rights of four men shot in separate incidents dating to 1997. All of the men were unarmed and shot in the back. Two of them died. The seven cases were detailed last month in a series of articles in The Washington Post which found that Prince George’s police shot and killed more people per officer than any other large police force in the nation from 1990 through 2000. Since 1990, Prince George’s officers have shot 122 people, 47 of them fatally. Top police officials have concluded that each of the shootings was justified.