Police were called, three floors evacuated and the package carefully placed in a vacant men's room. It turned out to contain a harmless greeting card in an oversize envelope, sent from overseas.
But the reaction reflected a panic sweeping Madison Avenue and elsewhere last week, as agencies and computer and airline companies strengthened safety measures to protect against an unknown and unpredictable assailant.
The bomb, postmarked Dec. 3 in San Francisco, exploded Dec. 10 at Mr. Mosser's North Caldwell, N.J., home, decapitating the newly promoted exec VP-general manager of Y&R Inc. and severely damaging his kitchen. His wife, their two children and others in the spacious house weren't injured.
It was the 15th in a string of attacks since 1978 by a bomber dubbed "Unabom" that have injured 23 and killed two. The package carried the return address of "H.C. Wickel" at San Francisco State University, but investigators at press time had traced no leads to that apparently fictitious name.
A virtual security dragnet enshrouded Y&R's offices, where camera crews swarmed, federal agents warily eyed visitors and X-ray machines were hastily installed in the mailroom.
Planned staff and media parties on Dec. 12 were canceled, a 24-hour hot line was set up to address staff concerns, and an agencywide memo warned employees not to open suspicious parcels.
"There's increased security, and we're taking additional precautions," said an agency spokesman, who like others declined to be specific. "The staff understands the need for further vigilance."
Those efforts may have been complicated by media coverage. The Associated Press last week identified Y&R CEO Peter Georgescu's posh Manhattan apartment building in a story noting increased security measures by New York City's police department.
As ad executives struggled for answers that would help explain the crime, FBI investigators probed several leads that would tie the bomber to Y&R.
Among them were recent news mentions of additional assignments awarded the agency by Digital Equipment Corp. and Xerox Corp.
And like some other targets, Mr. Mosser's promotion had been published in a major newspaper just before the attack.
FBI officials last week pored over Y&R and Burson client lists, Mr. Mosser's own files, and any records that would help pinpoint a motive for singling out the unassuming 50-year-old executive. They devised a high-profile strategy that sought leads from the public, and even posted a message on the Internet, alerting users they were "precisely the types of individuals" targeted by the bomber.
More than 3,000 calls to a special toll-free line-(800) 701-BOMB-offered tips or queries for officials.
"We have a task force of 25 investigators brainstorming about possible connections ... with other bombings," said Special Agent Bob Griego, of the FBI's San Francisco office, where the Unabom task force is based. "Anything's possible at this point."
Y&R said there was no suggestion by authorities that Y&R or its clients provided such a motive, but investigators wouldn't rule that out.
The lack of an easy explanation as to why Mr. Mosser was targeted made executives even edgier, and many companies, especially those in targeted industries, have stepped up safety measures.
When United Airlines then-President Percy Wood was injured by a mail bomb at home in 1980-believed to have come from the same culprit-the airline beefed up its corporate security measures, and continues to practice them today.
In the wake of the Mosser bombing, reports about suspicious packages also increased, just as the holiday mailing season had intensified.
"We have gotten more calls" about suspect packages, said a spokesman for the U.S. Postal Inspection Service. "We would much rather check a false alarm than go out and process a gruesome crime scene."
John T. Horn, senior managing director at Kroll Associates, a private investigation and security consultancy in New York, acknowledged a "momentary spike" in calls from worried executives, but said "it does not mean big business" for the firm, and urged against overreaction.
That didn't stop some, including a major technology client of Y&R.
"As you can imagine, we don't really want our name associated with this whole situation," said a spokeswoman there, who insisted on anonymity. "We're trying to low-key it all."
An IBM spokesman similarly noted "heightened awareness among security personnel and executives," but said the company is confident that current measures are adequate.
Several Time Warner executives, in Florida for the company's introduction of the Full Service Network last week said the bombing incident made them more than a little nervous.
Two of the executives said they've told their wives not to open any unexpected packages that arrive in the mail and to call the police if the parcels look particularly suspicious. Each agreed they were now uneasy with the thought of their names or photos appearing in newspaper stories.
"I live in New Jersey," said one, as if being in the same state where the bomb exploded was reason enough to be concerned.
Still, at least one high-profile executive said his lust for the limelight won't be lessened.
"It's a good business tool, and I'm not going to give it up," said Donny Deutsch, CEO of Deutsch, New York. "When someone tries to assassinate the president of the U.S., they don't stop being president."M
Contributing to this story: Pat Sloan, Scott Donaton, Alice Z. Cuneo, Christy Fisher, Jennifer Lawrence, Chuck Paustian, Ira Teinowitz and Melanie Wells.