MAIDAN SHAHR, Afghanistan — For months, reports have abounded here that the Afghan mercenaries who escort American and other NATO convoys through the badlands have been bribing Taliban insurgents to let them pass. 

Then came a series of events last month that suggested all-out collusion with the insurgents.

After a pair of bloody confrontations with Afghan civilians, two of the biggest private security companies — Watan Risk Management and Compass Security — were banned from escorting NATO convoys on the highway between Kabul and Kandahar.

The ban took effect on May 14. At 10:30 a.m. that day, a NATO supply convoy rolling through the area came under attack. An Afghan driver and a soldier were killed, and a truck was overturned and burned. Within two weeks, with more than 1,000 trucks sitting stalled on the highway, the Afghan government granted Watan and Compass permission to resume.

Watan’s president, Rashid Popal, strongly denied any suggestion that his men either colluded with insurgents or orchestrated attacks to emphasize the need for their services. Executives with Compass Security did not respond to questions.

But the episode, and others like it, has raised the suspicions of investigators here and in Washington, who are trying to track the tens of millions in taxpayer dollars paid to private security companies to move supplies to American and other NATO bases.