WASHINGTON — Border Patrol agents have racked up daily overtime at a cost of about $1.4 billion in the past six years while the number of arrests of illegal border crossers has fallen to the lowest level in nearly 40 years, an Associated Press analysis of agency records finds.

Since the 2006 budget year, the agency charged with stopping would-be illegal border crossers and smugglers from making it into the U.S. over land and sea borders has spent more than $1.4 billion on what is described as "administrative uncontrollable overtime," according to the data provided by the Border Patrol. In practical terms, agents average two hours a day in overtime.

That means agents can earn anywhere from 10 percent to 25 percent extra pay an hour for the first two hours of overtime, with the extra cash being steadily reduced every hour after that because of complicated overtime rules. Over the course of a year, an agent can earn about $15,000 more than the base salary, which for a more experienced agent is typically more than $60,000 a year. Agents are limited to $35,000 in overtime annually.