Citi Bike riders were left stranded on Wednesday after an outage at an Amazon data center knocked out service to the bike-share system during the height of the morning rush hour.

The disruption began shortly after 7:00 a.m., sparking complaints and confusion from monthly subscribers unable to unlock a bike. A spokesperson for Lyft, the Citi Bike parent company, said stations were beginning to come back online as of 9:20 a.m., though some riders continued to report issues.

Outside Bellevue Hospital in Manhattan on Wednesday morning, would-be commuters stood in front of a docking station fruitlessly trying to connect to the bikes with their phones.

“At morning rush? Are you kidding me?” lamented Paul Mindolovich, after learning of the outage from a customer service representative. “Take this off my yearly subscription.”

The Amazon Web Services (AWS) failure was tied to a data center failure in northern Virginia, according to Amazon. It affected several online services, including Slack, the Epic Games store, and a cryptocurrency exchange platform, that run on AWS servers.

For Citi Bike, which served an average of 70,000 riders per day last month, the unexplained disruption highlighted the ongoing challenges in linking a growing transportation system with an outage-prone cloud computing provider.

The incident on Wednesday morning was at least the third AWS outage this month to affect Citi Bike services, according to Lyft.

“It's certainly something that's of concern to us and we're investigating ways to build redundancy into the system,” said Jordan Levine, a company spokesperson. “You can trust us when we say we're as frustrated as our riders.”

Like many bike-share systems, Citi Bike relies on an app-based unlocking system that requires a connection to a server. The key fobs that were sent to early members also experienced issues, though they were not as widespread.

As stations began to come online Wednesday morning, bike-share users were not entirely buying the explanation from Lyft.

"If the subway didn't work and they told you it's because AWS is down, people would be pretty upset,” said Nick Hanley, a software engineer and daily Citi Bike user. “So I’m not sure that’s an acceptable answer.”

Hanley said he tried multiple docking stations in Chinatown, before eventually giving up and calling a cab to get to work.

“There's nothing worse than waking up in the morning and running around like a headless chicken trying to get your Citi Bike working and having them let you down,” he added.

This story has been updated with additional information from Amazon. Gwynne Hogan contributed reporting.