US Mail is flown both on passenger and cargo planes. That appears to mean that explosives

[pnwmom]

may have flown all over the country, possibly even on passenger planes, and escaped detection.

We are so lucky. But we need to be safer. I hope the TSA is on it.

https://blog.stamps.com/2017/06/09/what-is-air-mail/

Generally, Priority Mail, Priority Mail Express and First-Class Mail pieces will be shipped by air. However, these mail classes may also be shipped by ground depending on the originating and destination points—for example, if a destination is within driving distance. Postal trucks on the highway generally transport pieces that have to be shipped 500 miles away or less. For longer distances, your mail may be transported by freight railroad.

SNIP

USPS mail is carried on both passenger and cargo planes.

https://www.tsa.gov/travel/security-screening

Checked Baggage Screening

TSA screens approximately 1.3 million checked bags for explosives and other dangerous items daily.


FROM THE UK:
https://www.bbc.com/news/science-environment-33650713

A new system to resist terrorist bombs smuggled onto aircraft has been tested in dramatic experiments.
A device called FlyBag is designed to absorb the shockwaves and shrapnel caused by explosions.
If security fails and a bomb reaches the luggage hold, the idea is that the blast would be safely contained.
The trials - using old jets at Cotswolds Airport in Gloucestershire - showed that explosions on board caused no damage.

[roamer65]

1. You are correct, pnwmom.

You are right.

[PJMcK]

2. Wait a minute

When I check my luggage, it gets sent through the TSA's scanning X-ray machines to see if I have anything dangerous in my golf bag.

Don't they similarly screen the cargo and mail sacks? If not, that seems like a monstrously large hole in airline security.

[pnwmom]

3. The big question is whether any of these bombs ended up on a passenger plane.

Although the flight crew of a cargo plane would probably prefer their cargo to be screened, too.

[KentuckyWoman]

4. TSA is pure theatre

TSA doesn't do anything to stop terrorism on a plane. Not one thing.

[dixiegrrrrl]

5. FlyBag sounds like a good idea.

Not too sure how they fill all the mail and all the luggage into one, tho..must be pretty darn big.

[Blue_true]

6. Some packages that contain certain materials must always be shipped by ground.

I would doubt that the bomber used materials designations for the bombs, so some likely were flown on US Postal planes or commercial air planes (as cargo).