iStock/Thinkstock(NEW YORK) — The changing nature of the explosives being used in and around Austin has investigators scrambling for a break in the case, but police continue to reiterate their call for residents to be exceedingly cautious. Police have been warning residents about suspicious packages since the second and third package bombs detonated at residences in the Texas capital on March 12, but the nature of the warnings have changed as the types of explosives have evolved.The first three blasts stemmed from package bombs that were left in front of private homes. The fourth explosive is believed to have been detonated by a trip wire in a grassy area next to a road. And now investigators are working to determine if a blast at a FedEx distribution facility about 65 miles southwest of Austin is connected. According to police, who spoke with San Antonio ABC affiliate KSAT, a medium-sized box exploded at the facility in the early hours this morning.Possible warning signsThe general warning that Austin police have been stressing to residents is that they should report any suspicious items they see, “whether it be a package, a bag, a backpack or anything that looks out of place,” they wrote in a statement this morning. On March 17, one day before the fourth blast, the Austin police released a graphic with a picture of an example of a suspicious package. It highlighted several signs that should spark suspicion, including excessive postage, misspelled words or a badly written address. A note that the package is “Personal!” or confidential or has no return address should be red flags, too.Oily stains, strange scents, powder residue or protruding wires were also listed as indicators of a suspicious package. Specifics about the package deliveriesIn addressing the first three blasts, Austin police chief Brian Manley said they did not believe that the packages are being delivered by any formal mail services, including the U.S. Postal Service, FedEx, UPS or DHL.“They’re an average-size delivery box. They’re not exceptionally large,” Manley said on March 12, the day of the second and third blasts.The suspect or suspects did not ring the doorbell with the delivery of the first three packages, Manley said, so it is unclear how long the packages were on the victims’ porches before they saw and moved them.“What we know is when the victims have seen these packages on the front porch, they have gone out and handled them in some way or another and have had the explosions occur,” Manley said.“These devices can explode in many ways, either by being moved or being opened. So that’s why I want to reiterate the importance that if you see something that’s out of place, do not handle it, do not move it, do not touch it. Call us,” Manley said.“If you had a package at your home and you were not expecting a delivery, if the package delivered to your doorstep looks suspicious in any way, call 911 and report it. We will respond,” Manley said.Police received hundreds of calls about packages in the days after that warning.Copyright © 2018, ABC Radio. All rights reserved.