As many as 900,000 of Deutsche Telekom customers in Germany suffered network outages that might have been caused by hackers, the company and government security experts said on Monday, according to Reuters.

Deutsche Telekom customers had problems connecting to their networks on Sunday afternoon, as the outages impacted certain routers that dial into the network for phone, internet and TV access, the company said.

The number of customers that were affected decreased to about 400,000 after security measures were put in place. A software update was issued and the company asked affected customer to unplug their router for 30 seconds, the company said in a statement.

“There is no error pattern: some customers are experiencing temporary problems or very marked fluctuations in quality, but there are also customers for whom the service is not working at all,” the company said. “Based on the error pattern, we cannot exclude the possibility that the routers have been targeted by external parties with the result that they can no longer register on the network.”

Meanwhile, German security officials said the outages “obviously looks like the work of hackers" according to Reuters.

The company is still investing the issue, but said its technology experts have identified the problem as originating from how some routers connect to the network.

Vulnerable Routers

Hackers that handle Mirai malware, which turns IoT devices into denial of service tools, are exploiting a vital flaw that could be present in millions of routers, Ars Technica reported on Monday.

Routers used by customers of companies Deutsche Telekom and Eircom have already been labeled as vulnerable, researchers following the attacks say, according to Ars Technica. The flaws have been found in routers made by Zyxel, which is available in the U.S., Speedport and most likely other manufacturers as well.