The U.S. Postal Inspection Service is also investigating, officials said.
It was reported that two tests conducted on the package confirmed the presence of ricin, a highly toxic compound extracted from castor beans that has been used in terror plots.
Citing an unnamed law enforcement official briefed on the matter, The New York Times reported that letters were sent "in recent days" to the White House and to federal agencies in the U.S. state of Texas. The package was sent to the White House and addressed to President Trump.
The White House and Secret Service did not immediately return a request for comment.
"At this time, there is no known threat to public safety", the Federal Bureau of Investigation said in its statement.
The New York Times reports that not only was a ricin letter addressed to the president, but other letters containing the deadly substance were sent to "federal agencies in Texas" and perhaps others as well. Ricin can cause death within 36 to 72 hours from exposure to an amount as small as a pinhead.
There have been numerous incidents involving envelopes mailed with ricin to USA officials.
In 2018, a Utah man was indicted on making threats of mailing ricin to President Donald Trump and the Pentagon.
Ricin is produced by processing castor beans. Richardson was sentenced to 18 years in prison and ordered to pay $367,000 in restitution.