A law enforcement official said the envelope and printed address labels on the packages sent to Mr. Biden and Mr. De Niro were similar to those sent to the CNN offices in New York and to former President Barack Obama, former Secretary of State Hillary Clinton and others. An X-ray showed the package sent to Mr. De Niro contained a device that seemed to be a pipe bomb.

Federal, state and local investigators in New York, Washington, Florida and Los Angeles have all joined forces in the rapidly widening case, which has sent a shock through the nation’s political and news media establishments.

Though investigators initially believed that some of the packages were delivered by hand or courier, they have now concluded that all 10 were likely sent through the mail, a person briefed on the matter said.

The Postal Service records images of mail that comes into its system. As part of the inquiry, officials have been searching those images in an effort to determine where the packages originated, as well as to identify and catch any other possible explosive devices.

In addition to conducting a forensic examination of the packages and explosive devices, federal authorities were seeking to determine where the envelopes, mailing labels and bomb components were purchased.

Several law-enforcement officials said that investigators would likely obtain data from cell towers in the areas where they believe the packages were mailed. The data from each area could then be cross-referenced to create lists of phone numbers — and, ultimately, phone users — who were in the areas around the times the packages were mailed, the officials said.