Karen Bleier/Agence France-Presse — Getty Images

At 2 p.m. Eastern time on Wednesday, all television channels and radio stations in the United States were supposed to be interrupted by a brief test of the nation’s Emergency Alert System. But, like most tests, some passed and some failed.

Beginning at 2:01 p.m., viewers and listeners in many states said they saw and heard the alerts at the scheduled time, but others said they did not. There was no immediate explanation for the discrepancies, but that was one of the purposes of the test — to find out how well the system would work in an actual emergency.

Certainly, viewers and listeners have grown accustomed to hearing the tones and reminders — “this is just a test” — when the systems are activated locally each week by broadcasters. But government officials said the national system had never been tested before as a whole, nor had it been used in an emergency, allowing the president to address the public.

Many of the reported failures affected cable and satellite television subscribers, and some were quite puzzling. Some DirecTV subscribers said their TV sets played the Lady Gaga song “Paparazzi” when the test was under way. Some Time Warner Cable subscribers in New York said the test never appeared on screen. Some Comcast subscribers in northern Virginia said their TV sets were switched over to QVC before the alert was shown.

Karen Bleier/Agence France-Presse — Getty Images

In some cases the test messages were delayed, perhaps because they were designed to trickle down from one place to many. A viewer in Minneapolis said he saw the message about three minutes late. A viewer in Chattanooga, Tenn., said she saw it about 10 minutes late.

In Greensboro, N.C., a local reporter saw the alert on all the cable news channels but on none of the local broadcast networks. In Los Angeles, some cable customers said the alert lasted almost half an hour.

Many other viewers and listeners reported that the alert arrived right on time at 2 p.m. Eastern. It halted digital video recorder playback in some households and surprised radio listeners in their cars.

The Federal Communications Commission and the Federal Emergency Management Agency were the two agencies in charge of the test.

“We always knew that there would probably be some things that didn’t work and some things that did,” a FEMA official said an hour after the test. The official spoke on the condition of anonymity because the agencies had not publicly acknowledged the glitches yet.

In a statement around 2:40 p.m., the agencies said they were collecting data about the results of the test.

“This initial test was the first time we have tested the reach and scope of this technology and additional improvements that should be made to the system as we move forward,” the statement read. “Only through comprehensively testing, analyzing and improving these technologies can we ensure an effective and reliable national emergency alert and warning system.”

The agencies said they looked forward to working with media companies to “improve this current technology and build a robust, resilient and fully accessible next generation alerting system that can provide timely and accurate alerts to the American people.”

Before the test, officials said they expected it to last about 30 seconds as it digitally rippled across the country. Government agencies and media companies sought to spread the word about the test so that it did not sneak up on, and potentially scare, the public.

Michael Powell, the head of the Cable and Telecommunications Trade Association, wrote on Twitter, “No one wants a ‘War of the Worlds’ sequel!”

During the test, the on-screen text set up by the government read, “This is an Emergency Action Notification.” It did not specify that the notification was only a test because officials wanted the test to duplicate actual alert conditions “as closely as possible,” according to an F.C.C. planning document. But voiceovers and other on-screen graphics indicated that there was no need for alarm.

Satellite distributors like DirecTV and cable distributors like Comcast participated along with broadcast stations. Internet connections were not included in the test, which led some critics to question how comprehensive the alert system could be.

Notifications about the test were hard to miss ahead of time — though some people surely were surprised by it anyway. Public service announcements and graphics ran on local stations, messages appeared on cable customer bills and Web sites and news segments informed people about the plan.

What is now called the emergency alert system was authorized in 1951 by President Harry S. Truman. It was intended to inform Americans about any impending nuclear attack and was called Conelrad, short for “Control of Electromagnetic Radiation.” The system was superseded by the emergency broadcast system, which was used primarily for local weather alerts and was replaced by the current emergency alert system in 1997.

After the terrorist attacks of Sept. 11, 2001, President George W. Bush addressed the nation through the major television networks without activating the emergency alert system. The networks were able to transmit Mr. Bush’s statements live on their own.