WASHINGTON — Homeland Security officials on Friday released revised instructions for when Border Patrol agents can use deadly force, but questions remain over how rigidly the new rules would be enforced by agents in the field.

The guidelines are intended to respond to concerns raised in a scathing internal report that found Border Patrol agents had deliberately stepped in front of moving cars apparently to justify shooting at drivers, and had fired in frustration at people throwing rocks from the Mexican side of the border.

At least 20 people have been killed in confrontations with Border Patrol agents since January 2010. In at least nine cases, agents said they were pelted with rocks before they responded with lethal force. Six people, including three minors, were standing across the border in Mexico when they were shot to death.