A sixth, unexploded package bomb was discovered today at the FedEx facility in Austin, Texas, two sources briefed on the investigation told ABC News.
The package, which is intact, was discovered by FedEx workers, the company said in a statement. It's believed to have been sent by the same individual who mailed a package bomb that exploded at a FedEx facility near San Antonio this morning, officials said, and could yield major clues in the hunt for the serial bomber terrorizing Austin.
"We have also confirmed that the individual responsible [for Tuesday's package bomb] also shipped a second package that has now been secured and turned over to law enforcement," FedEx said in its statement but did not reveal which of its facilities the package was discovered in.
The two sources confirmed to ABC News the undetonated package was discovered at an Austin facility, bringing the total number of devices believed to be connected to the investigation to six.
"We have provided law enforcement responsible for this investigation extensive evidence related to these packages and the individual that shipped them collected from our advanced technology security systems," FedEx said. "The safety and security measures in place across the FedEx networks are designed to protect the safety of our people, customers and communities, and to assist law enforcement as appropriate."
The string of explosions in Austin, which left two dead and four injured, has the city on edge and sparked hundreds of law enforcement to descend on the area.
The FBI has discovered a link between packages involved in the Austin bombings and a mail delivery office at a strip mall in the neighboring city of Sunset Valley. Police cordoned off the area and were combing through the office today.
In his first public comments on the Texas bombings, which started March 2, President Donald Trump said the federal government is doing everything it can think of to catch the culprits.
"These are sick people and we need to find them immediately," Trump told reporters at the White House.
Today's explosion in the San Antonio suburb of Schertz occurred about 12:45 a.m. at a FedEx facility. About 65 miles away, two men were injured Sunday night in the fourth bombing in 17 days to rock the Texas capital.
Austin Police Chief Brian Manley told the Austin City Council this morning that the package that exploded in Schertz originated at a facility in Austin that has been closed.
"A package had been traveling along an automated conveyor when it exploded," Schertz Police Chief Michael Hanson said at a news conference outside the Schertz FexEx facility this morning.
Hanson said the injured worker complained of "ringing in her ears," was treated at the scene and released.
Earlier today, police told ABC San Antonio affiliate KSAT-TV that the medium-sized box that exploded contained nails and metal shrapnel.
Jim McClusky, a spokesman for FedEx, confirmed in a statement to ABC News that "a single package exploded" and that one worker was treated for minor injuries.
There were 75 people in the building at the time of the explosion, authorities said.
"This is a fluid investigation and we want the general public to know their safety is our No. 1 priority," said Frank Ortega, ATF acting assistant special agent in charge.
He said the ATF's National Response Team and bomb-sniffing dogs were searching the facility for other possible explosives.
Neither Ortega or FBI assistant special agent James Smith, who both spoke at the news conference, would say if investigators definitively believe the explosion is connected to the Austin bombings.
Manley said investigators from his department, the FBI and the ATF were working to determine whether the explosive is the work of a serial bomber or bombers who have terrorized Austin since the beginning of the month.
He said it was too early to say if the explosive package in Schertz has any connection to the bombs that have gone off in Austin, including that most recent one on Sunday night that was triggered by a tripwire and injured two men.
"We've seen an evolution in the type of devices, the one in Schertz involves a delivery service. What we are seeing here is unprecedented," Manley told the city council.
He cautioned residents to pay close attention to any suspicious device whether it be a package, a bag, a backpack "or anything that looks out of place" and advised residents to immediately call 911 and stay clear of the suspicious items.
Manley said Monday at a news conference on the Austin explosions, "clearly we are dealing with a serial bomber."
Austin residents have been on edge for a month after the series of bombings. The first three bombings were packages, apparently hand-delivered and left on porches of homes before exploding when picked up by residents, police said.
The fourth explosion took place Sunday night. Two men, ages 22 and 23, were injured in that blast, which was caused by a device using a tripwire on a sidewalk in the Travis Country neighborhood of Austin.
ABC News' Jack Date contributed to this report.