The bus manufacturer has performed inspections and tests but has not found a root cause. More testing will continue this week, Metro said.

WASHINGTON — Metro is not sure when 164 buses pulled from service due to safety concerns may be cleared to return to the road, so some riders should expect delays to continue.

The investigation into why two of the 2015/2016 New Flyer compressed natural gas buses unexpectedly shut off while in motion remains open, Metro spokeswoman Sherri Ly said, and all of the buses remain out of service about a week after the second shutdown occurred.

“Additional testing will continue this week, including test runs of buses equipped with special data recorders to attempt to gather additional information,” Ly said in an email.

The bus manufacturer has performed a number of inspections and tests but has not found a root cause.

“We are proceeding with customer and employee safety as our highest priority, and there is currently no timetable as to when these buses may be returned to service. There are no significant customer impacts to report, as reserve buses have been placed into service and equipment has been redeployed to cover most scheduled trips,” Ly wrote.

Metro’s Twitter account dedicated to the bus system has reported delays on a number of routes tied to the buses that are out of service, because Metro does not have enough reserve buses to cover all of the trips. Many of the reserve buses are older models.

Among the routes specifically noted so far: 3Y, 22F, 29G, S91, 18H, 18J, 18P, 22B, 16B, 16H, 16J, 16X, 17L, 17B, 17M, 8S, 8W, 8Z, 16G, 4A, 7M, 7F, 7W, 7Y, A4, NH2, W5, 11Y, 32, P18, W14, 23B, 28F and 28G.

In some cases, only one or two scheduled trips might have been skipped on one of those routes, while there have been somewhat-larger impacts on other routes.