FBI officials are investigating an anonymous threat against one of the Mississippi River bridges in Memphis, but Warren County officials say they have not been briefed of any local threat.

This week, the FBI said they had received an anonymous, unsubstantiated threat against one of the bridges over the Mississippi River in Memphis and had sent a bulletin out to Tennessee and Arkansas law enforcement agencies.

Warren County Emergency Manager John Elfer said his office had received no bulletin detailing a threat against the Interstate 20 or U.S. 80 bridges over the Mississippi.

Warren County does participate in a Department of Homeland Security program called “If You See Something, Say Something,” Elfer said.

“If the public sees something that doesn’t look right, they need to call 911 immediately and get as much information as they can — vehicle descriptions, individual descriptions and what type of activity was going on,” Elfer said.

Drivers reporting activity should not approach any suspicious person on or near the bridge, Elfer said.

About 30,000 vehicles cross the Interstate 20 bridge over the Mississippi every day, according to the Mississippi Department of Transportation. About 950 rail cars cross the U.S. 80 bridge every day, bridge Superintendent Herman Smith said.

“We have right at about 29,000 cars per month,” Smith said.

Kansas City Southern Railroad holds the lease to cross the bridge, Smith said.

The threat against the Memphis bridge claims that the terrorist group known as the Islamic State had activated orders “to blow up the Memphis-Arkansas bridge on an unknown date, activating ISIS terror cells in the United States,” according to the bulletin received by Tennessee and Arkansas officials.

Two cross-country routes traverse the Mississippi River at Memphis — Interstates 40 and 55 — and both remained open. Each bridge carries about 40,000 vehicles daily.

The FBI advisory, sent Monday, cited a threat to the “Memphis-Arkansas” bridge from an Islamic State cell in Memphis. The I-55 bridge is known as the Memphis and Arkansas Bridge but officials said Tuesday the threat was interpreted as applying to both bridges.

Both bridges remained open Wednesday morning.

Highway officials reviewed their contingency plans for rerouting traffic and asked its workers to remain vigilant.

The Arkansas Department of Emergency Management said the threat had not reached a level that would require it to put anyone on stand-by. Spokesman Rick Fahr said threats that include specifics happen rarely.

The Associated Press contributed to this report.