Throughout his campaign, President-elect Donald J. Trump said border security would be one of his highest priorities. As he prepares to take office, he will find that many of the problems seem to come from within.

“It does absolutely no good to talk about the building of walls or tougher enforcement if you can’t secure the integrity of the immigration system, when you have fraud and corruption with your own employees,” said an internal affairs official at the Department of Homeland Security who spoke on the condition of anonymity.

Although Homeland Security employees who have been caught taking bribes represent less than 1 percent of the more than 250,000 people who work at the department, investigators say the bribes and small numbers of people arrested and charged with bribery obscure the impact corruption can have on border security and immigration enforcement.

“Any amount is bad, and one person alone can do a lot of damage,” said John Roth, the inspector general at the Department of Homeland Security. “It doesn’t have to be widespread.”

Law enforcement experts say the bribing of border and immigration agents is not surprising. As security along the border has tightened with the addition of fences, drones and sensors, drug cartels and human smugglers have found it increasingly more difficult to operate.