In 1974 a case of Domestic Terrorism hits the Gulf Building in Downtown Pittsburgh and has gone unsolved to this day.

Yes when I was a young cub scout in late 60's you could actually go up to the top of the Gulf building in Downtown Pittsburgh and they had an observation deck on top which we all checked out . At the time the Gulf Building was the tallest building in Pittsburgh  with its high powered blue or red beacon depending on what weather was doing and when it flashed meaning weather was changing would spell in Morse code welcome to Pittsburgh  this was before they built the US steel Tower also known as the Rusty Nail. for it rusty looking  Corten Steel. was built in 1970's before the US Steel tower was erected when we went to visit our aunt Peg who lived on 3rd floor of a tenamant on 49th street in Lawrenceville we could see the Gulf Building.

https://en.wikipedia.org/wiki/Gulf_Tower

That easy carefree time came to an end on June 13th 1974 around 9:30 Pm when the  Guard watching the desk gets a call telling him he had 20 minutes to evacuate the building there was a bomb  and several minutes latter there was a massive explosion  on the 29th floor  luckily no one was hurt and emergency officials who had been checking out the building had just cleared the area before it went off .

https://news.google.com/newspapers?nid=1979&dat=19740611&id=lGgiAAAAIBAJ&sjid=9KkFAAAAIBAJ&pg=4060,1913337

https://news.google.com/newspapers?nid=1291&dat=19740614&id=5cMPAAAAIBAJ&sjid=FI0DAAAAIBAJ&pg=3866,5946231

https://news.google.com/newspapers?nid=1129&dat=19740614&id=ZctRAAAAIBAJ&sjid=Y20DAAAAIBAJ&pg=5613,1841833

The Gulf Building was targeted supposedly by The Weather Underground  because the famous Gulf Oil Company was headquartered there and the Weather Underground held them responsible for supposed deaths they caused to native Africans  during there oil explorations.

The building from that day on was sealed off no more of the public just walking in and the observation deck was closed forever .

 The Pittsburgh Police and FBI would investigate along with Allegheny county Detectives  but no suspects where developed or arrests ever made . Of course back then Closed Circuit cameras where still in there infancy and few where deployed and most did not record. To my knowledge the Gulf building did not have them.  I suspectsome one posing as a cleaner or contractor planted the explosives and since no one checked in back in those days the Gulf Building was an easy open target.