So far, intelligence officials said, all of the indications are that it was simply an act of extortion by the group, which first began to deploy such ransomware last August, and is believed to operate from Eastern Europe, possibly Russia. There was some evidence, even in the group’s own statements on Monday, that suggested the group had intended simply to extort money from the company, and was surprised that it ended up cutting off the main gasoline and jet fuel supplies for the Eastern Seaboard.

The attack exposed the remarkable vulnerability of a key conduit for energy in the United States as hackers become more brazen in taking on critical infrastructure, like electric grids, pipelines, hospitals and water treatment facilities. The city governments of Atlanta and New Orleans, and, in recent weeks, the Washington, D.C., Police Department, have also been hit.

The explosion of ransomware cases has been fueled by the rise of cyberinsurance — which has made many companies and governments ripe targets for criminal gangs that believe their targets will pay — and of cryptocurrencies, which make extortion payments harder to trace.

In this case, the ransomware was not directed at the control systems of the pipeline, federal officials and private investigators said, but rather the back-office operations of the firm. Nonetheless, the fear of greater damage forced the company to shut down the system, a move that drove home the huge vulnerabilities in the patched-together network that keeps gas stations, truck stops and airports running.

A preliminary investigation showed poor security practices at Colonial Pipeline, according to federal and private officials familiar with the inquiry. The lapses, they said, most likely made the act of breaking into and locking up the company’s systems fairly easy.