WASHINGTON - The mini-helicopter that flew into the US capital this month has exposed a security problem as military radar are not designed to distinguish slow-moving, small craft from flocks of birds, officials have said.

The gyrocopter that violated Washington’s no-fly zone and landed on the US Capitol lawn on April 15 was not picked up by radar because the systems are set up to ignore slow, small and low-flying objects that can include kites, birds or balloons, officials said.

“Identifying low altitude and slow speed aerial vehicles from other objects is a technical and operational challenge,” Admiral William Gortney, head of the North American Aerospace Defense Command (NORAD), told lawmakers.

The flight of the small gyrocopter by a protester trying to draw attention to campaign finance reform has raised alarms about security in Washington and defenses against a possible drone assault.

Lawmakers on the House Oversight and Government Reform Committee grilled Gortney and senior officials about how to prevent a potential attack by a drone or a mini-helicopter.

Gortney acknowledged that the pilot could have crashed his gyrocopter into the Capitol, which houses the US Congress whose 535 members were in session at the time of the incident.

The military is working on a technical solution that could address the gap, Gortney said, including the JLENS system that relies on aerostats to identify drones or other small-scale aircraft.

But that system is still being tested and is not yet operational, he said.

One lawmaker expressed frustration that America could not find a way to counter such a low-tech threat.

“We can put a man on the moon, fly a rover to Mars, and we can’t stop a postman” coming in with a gyrocopter, said Mark Meadows, a Republican from North Carolina.

The incident involving the mini-helicopter two weeks ago revealed the potential danger posed by robotic aircraft, lawmakers said, especially as drones become increasingly prevalent for commercial and recreational flights.

“It’s not about gyrocopters only, it’s about drones,” said Matt Cartwright, a Democrat from Pennsylvania.

In January, an intelligence agency employee lost control of a hobby drone and crashed it on the White House lawn, prompting a Secret Service investigation.

Since September 2014, the Federal Aviation Administration has approved 250 authorizations for civilian drone flights, allowing the use of unmanned aircraft for filmmakers, farmers and industry.