A sign at the Forest Glen station lists outages and delays. Regularly scheduled maintenance added to the disruption caused by computer glitches Saturday and Sunday. (Katherine Frey/THE WASHINGTON POST)

Sen. Barbara A. Mikulski lashed out at Metro on Monday, decrying the recent run of safety problems, even as the transit agency said it still could not explain what caused a computer glitch that forced the shutdown of the system twice over the weekend.

“Many of my constituents are really disturbed,” Mikulski (D-Md.) said at a previously scheduled media event to talk about new safety standards for Metro and other systems. “The airwaves are hopping today. They’ve been hopping over the weekend. E-mails have been coming in. Some volcanic, some even more so.”

Mikulski, a harsh critic of Metro since the 2009 Red Line crash that killed nine people, said recent incidents have left her constituents “terrified.”

“When you get on the Metro and you’ve got kids with the stroller, you wonder — if the cars open and you’ve got to jump down and evacuate quickly, what will happen to you?” she said. “If you’re a senior trapped in a car with 100-degree heat and your pacemaker starts to race, you wonder what’s going to happen to you.”

Metro officials said the agency will continue to station extra employees at key spots throughout the system in case there is a recurrence of the glitch that halted service on all five rail lines over the weekend.

Mikulski and other officials had gathered Monday afternoon at Metro’s operations center in Prince George’s County to address recent changes in federal law that will create uniform safety regulations for subway and light-rail systems nationwide.

Metro’s weekend troubles ended up being a focus of the event.

The top Metro official at the event, Deputy General Manager Dave Kubicek,said officials still don’t know what caused train monitoring systems to go dark twice in a 12-hour period. And it was not clear when they would have answers.

He said officials do not think the blackout was caused by a cyberattack. “It’s not a network-type system,’’ he said. “It’s a closed-loop system. That’s why we don’t believe it’s from outside.”

At the same time, officials continued to downplay the role the train monitoring system plays in daily Metro operations.

“It’s a system that’s critical to the operation of trains, but it is not a safety-critical system,” Metro spokesman Dan Stessel said.

But some safety experts say Metro’s train monitoring system does play a critical role in its operations.

“These computer systems are basically the brain and the eyes and the ears of the operator,” said Najmedin Meshkati, a University of Southern California systems engineer who studies transportation safety. “If they go blind, you are having someone in control with insufficient information. You can’t have blind and deaf operators.”

Robert Sapitowicz, vice president of marketing and communications for ARINC, the Annapolis-based company behind the system, said he could not talk about the investigation because Metro officials had asked that all calls be referred to them.

Meanwhile, officials from the National Transportation Safety Board said they do not plan to open an investigation of the outage. The Tri-State Oversight Committee, which monitors the rail system, expects to receive a preliminary report on the incident this week.

Meshkati said he also was concerned that the computer failure was a problem that affected both of Metro’s operations centers, even though two exist so that one can serve as a backup if the other malfunctions. “Something fell through the crack here,” he said.

Stessel said passenger safety was not compromised by the outages and that officials shut down the system as a precaution. As part of that, train operators were told to move their trains to the nearest stations, where they remained until controllers could restart the system.

Legislators said the safety standards recently signed into law as part of the larger transportation bill would go a long way toward making rail systems safer by beefing up federal oversight.

“While we’ve made progress, we know from the incidents of the last couple of days and last couple of months that we have work to do,” said Rep. Chris Van Hollen (D-Md.).

July has been a tough month for the nation’s second-busiest transit system. Riders were already grumbling about a fare increase that kicked in July 1. Two days later, a botched evacuation just outside of the College Park station on the system’s Green Line again raised questions about the authority’s ability to manage passengers during an emergency. That incident was followed by the July 6 derailment of a Green Line train near West Hyattsville. Then on Monday, a temporary shutdown of the Dupont Circle station left some riders fuming.

Mike Robinson, a retiree, has been riding Metro since it opened.

“I can recall when it was reliable, and new, and safe,” he said as he waited for a Shady Grove-bound Red Line train. “Today, it is none of those things.”

“I’m always concerned I’ll be in a car with some kind of glitch” said Robinson, who was stranded on a train last summer. “I never did hear an explanation for that,” he said. “We’re generally left in the dark about what’s wrong or why.”

With each service delay, some riders just wish Metro would provide more information, and do it more quickly. “They just say, ‘This is it,’ ” said Jon Bush. “They don’t tell you why, and they wait too long to say it.”

Of Metro’s communication skills, Bush had one positive thing to say: “They’re better than Pepco.”

Ted Trautman contributed to this report.