"As you can imagine, the pain of not knowing where a loved one is can be unbearable," Mittendorff's husband, Steve, said in tears during a news conference Tuesday at a Fairfax County firehouse.
Mittendorff, 31, was last heard from in text messages last Wednesday,
family and friends have said on social media.
She was reported missing Friday after failing to show up at work, her father, Robert Clardy, said in a Facebook post.
Mittendorff's 2009 Mini Cooper was found by a U.S. Park Ranger Saturday night in a parking lot near a popular hiking spot at Shenandoah National Park, authorities said.
The search is focused on an area called White Oak Canyon Trail, near where her car was found, authorities said. About 100 state, federal, and volunteer search and rescue teams combed the area Monday, and efforts resumed Tuesday morning.
There's "no evidence to indicate that her disappearance is suspicious in nature," Virginia State Police spokeswoman Corinne Geller said.
At the news conference Tuesday, Fairfax County Fire and Rescue Chief Richard Bowers called for the public's help in locating Mittendorff -- a "motivated and dedicated" firefighter and paramedic for three years who was "respected and well-liked" by her colleagues.
"It has been almost a week and we are saddened that Nicole is not back with us," he said.
Bowers asked for a moment of silence for Mittendorff and said a prayer vigil will be held at the firehouse on Wednesday evening.
Mittendorff is 5 feet 6 inches tall, weighs about 125 pounds and has blonde hair and green eyes.