WASHINGTON — Republican leaders in Congress are launching an investigation into whether top police officials in Washington, D.C., engaged in a cover-up of crime statistics to make it seem as if illegal activity in the capital city was decreasing over the last three years. The House Oversight Committee announced on Monday it would open an inquiry into the Metropolitan Police Department and whether it deliberately manipulated data to show lower crime rates. As part of the inquiry, the panel is requesting access to all documents and materials detailing the decision-making behind the department’s crime reporting process.