All Metrorail service was suspended early Sunday for the second time in a 12-hour period after a computer problem arose again.

“There was another computer issue between 12:30 and 1:00 a.m.” said Metro spokesman Dan Stessel in an e- mail. He said the trains operating at that time were held at stations with the doors open until the system was restored.

It is not clear what had caused the problems Saturday afternoon or Sunday morning. However, Stessel said both incidents were similar. The first involved the ability to track trains as they traveled through the system.

He said that all trains were moving again as of 1 a.m., while technicians remained on the scene working to determine the source of the problem.

Metro normally closes around midnight, but on Friday and Saturday nights the system remains open until about 3 a.m.

Service was suspended for more than 30 minutes Saturday after a computer problem left controllers unable to track trains as they moved through the system.

In the Saturday afternoon incident, service was suspended for more than 30 minutes. Stessel said the problem began a little after 2 p.m. when controllers in the Operations Control Center reported that the computer system that enables them to monitor trains had gone dark. About 44 trains were operating at the time. All trains were told to hold at the nearest station until the problem could be resolved, Stessel said.

Twitter lit up with passengers’ complaints about the delays.

Service was restored to all lines just before 3 p.m., but Stessel said officials were still trying to determine what caused the problem. Although the shutdown was not unprecedented, such incidents are rare, he said.

“It is operating normally now, and it’s stable,” he said in the afternoon. But there was no guarantee the problem would not recur, Stessel said. “Technicians will remain on the scene working to determine what the cause was,” he said.

The computer problem coincided with service delays related to scheduled track work on all five lines. That work was originally scheduled to be done the previous weekend but was postponed because emergency repairs had to be made to a section of the Green Line where a train carrying 55 passengers derailed July 6. No injuries were reported in that incident.

Metro has had a difficult July.

A couple of days after a fare increase went into effect July 1, Metro officials were accused of mishandling the evacuation of a Green Line train that broke down outside the College Park station, stranding hundreds of passengers in sweltering rail cars. That incident was followed by the derailment, which officials say was heat-related.

In response, Metro officials said they will revise evacuation procedures as well as the policy for operations in extreme heat.

Staff writer Tessa Muggeridge contributed to this report.