The remaining members of a Charlotte-based N.C. National Guard unit are returning from firefighting duty Tuesday or Wednesday, after four guardsmen were killed and two wounded in a plane crash Sunday in South Dakota’s Black Hills.

The cause of the accident, which resulted in the crash of a C-130 plane specially outfitted for spraying flame retardant at low altitudes, remains under investigation by a military board. Lieutenant Colonel Robert Carver said discussing possible causes of the crash would be “rumors and speculation.”

Lt. Col. Paul Mikeal of Mooresville, Master Sgt. Robert Cannon of Charlotte, Maj. Joe McCormick of Belmont and Captain Major Select Ryan Scott David of Boone were killed in the crash, according to family members and military officials.

Officials did not release the names of the two injured airmen who survived the crash because doing so is against protocol, they said. The two are still in serious condition, but officials said they are “guardedly optimistic” that they’ll recover.

The voice of the wing commander, Brigadier General Tony McMillan, wavered as he spoke to reporters Tuesday, in front of a memorial and a decommissioned C-130 plane at the unit’s base at Charlotte Douglas International Airport.

“We continue to grieve, but we are ready for any mission that may come our way, knowing we have four new wingmen watching over us,” he said.

The unit returned to the U.S. in March after spending several months deployed in Afghanistan. Many of the guardsmen in the unit have been deployed to war zones multiple times. There are about 30 other wingmen in the unit who went to South Dakota to fight the wildfires, and they are expected to arrive in Charlotte later today or Wednesday in the unit’s two remaining planes.

City and state flags will fly at half-staff Tuesday across North Carolina, in tribute to the four Charlotte-based Air National Guard members who were killed.

Josh Marlowe of Shelby was seriously injured in the crash and in a South Dakota hospital, his mother-in-law told the Observer. The sixth airman’s identity was not immediately known.

The C-130 plane, from the N.C. Air National Guard’s 145th Air Wing, was pressed into service due to wildfires that have burned thousands of acres and destroyed hundreds of homes in Colorado, Wyoming and South Dakota.

The Air Force grounded the seven other C-130 planes Monday that were fighting the wildfires, but Denver TV station KMGH reported Tuesday morning that the planes had been cleared to fly again. The station tweeted that C-130 firefighting missions will resume Tuesday.

Air Force officials said the cause of the crash remains under investigation.

But a blogger at the website Wildfire Today cited information from the U.S. Forest Service indicating a plane ahead of the C-130 experienced a “severe downdraft” while approaching the area where the planes had been assigned to drop fire retardant.

The Air Force grounded seven other firefighting C-130s in the wake of the crash Monday, removing critical equipment from the skies during one of the busiest and most destructive wildfire seasons ever in the West.

The plane that went down, a C-130H3, was manufactured in 1993, according to First Lt. Michael Wilber, a spokesman for the N.C. National Guard.

The maintenance crew found no mechanical deficiencies on the plane the last time it was on the ground, Wilber said. It was one of three planes sent from Charlotte.

All six on the flight were experienced crewmen who had drilled in fire missions, according to Lt. Col. Robert Carter of the N.C. Air National Guard.

The Charlotte-based Air National Guard crews were helping the Colorado, Wyoming and California National Guard units battle the Waldo Canyon and Flagstaff fires in Colorado; the Arapaho fire in Wyoming; and the White Draw blaze in South Dakota.

The C-130 planes were carrying the Modular Airborne Fire Fighting System (MAFFS), a self-contained firefighting system owned by the Forest Service. MAFFS can discharge 3,000 gallons of water or fire retardant in a matter of seconds.

Those planes, which made multiple flights in recent days, typically aren’t deployed unless the rest of the Forest Service’s firefighting fleet is occupied.

Obama: ‘They are heroes’

President Barack Obama said the firefighting crews “put their lives on the line every day for their fellow Americans.”

“They are heroes who deserve the appreciation of a grateful nation,” Obama said. “The crew of this flight – along with their families and loved ones – are in our thoughts and prayers.”

Gov. Bev Perdue said in a statement that the “tragic loss underscores the risks and sacrifices our servicemen and women make on a daily basis. Whether home or aboard, they leave their families to keep us safe and protect our freedom.”

Perdue encouraged North Carolina residents to join state officials in flying flags at half-staff.

Charlotte Mayor Anthony Foxx said Tuesday morning that all flags on city property will be flown at half-staff.

U.S. Rep. Patrick McHenry, R-N.C., who represents Mikeal’s district in Congress, said, “Mikeal and his crew fought through dangerous conditions, trying to save the homes and lives of thousands of Americans. His courage and heroism will not be forgotten.”

About 1,400 men and women are based with the Charlotte National Guard unit. Most of them are from the Carolinas. The 145th Air Wing has ten C-130 aircraft.

Second run of the day

The two firefighting planes from the Charlotte unit, along with a third craft used for equipment and supplies, flew to the Colorado base Saturday.

A spokesman for the 145th Airlift Wing said its aircraft were scheduled to move Monday to a base in Wyoming, so they could be closer to the fire.

Mikeal’s plan crashed about 8:30 p.m. MDT Sunday while making its second run of the day in fighting the White Draw fire, which has charred more than 4,200 acres in an area about 90 miles southwest of Rapid City.

Officials said the aircraft disappeared from radar. Details of the crash were not available. The sheriff’s office in Fall River County, S.D., told the Rapid City (S.D.) Journal that a rescue helicopter had been able to land near the crash scene.

The terrain of the crash site is “very, very rugged, straight up and straight down cliffs,” said Frank Maynard, the Fall River County emergency management director.

‘Difficult mission’

The C-130, a versatile four-engine plane built by Lockheed Martin, has been widely used for fighting fires and for transporting military cargo and personnel.

Although the plane has a generally low accident rate, some older model C-130s have gone down battling fires.

In June 2003, a C-130A lost both its wings and crashed while fighting a wildfire in northern California. All three crew members died.

A C-130A also crashed in September 2000, while dumping water over a forest fire in southeastern France. Two of the four crew members were killed.

Carter, of the N.C. Air National Guard, said this is the first crash of a C-130 equipped with a MAFFS unit in the 40 years the C-130s have been fighting fires.

But battling fires from the air is a dangerous business, says Gene Rogers, a wildfire consultant from Oregon. Pilots must contend with smoke, hilly terrain, convection drafts and low-altitude turbulence.

Over the past six decades in the United States, there have been an average of about 1.5 crashes a year involving large planes on firefighting missions, Rogers said.

“Flying a large aircraft anywhere from 150 feet to 300 feet over undulating terrain is outside the box of traditional pilot expectations,” said Rogers, who served for 34 years with various federal agencies, working on fire suppression issues. “It’s a difficult mission.”

Staff writers Mark Washburn, Ely Portillo and Elisabeth Arriero, researchers Marion Paynter and Maria David, Washington Bureau reporter Franco Ordoñez and the Associated Press contributed.