I don't know about additional explosives, but I do believe the BATF helped set up the bombers. Two BATF undercover informants spoke on TV after they were not permitted to testify in court. Both said that the BATF aided and enabled the bomb makers. It was intended that they would then jump in to "save the day" at the last moment. However, they are as incompetent as they are corrupt and at the last moment, they lost contact with their bomber. It was just like Waco, intended to get good press for the BATF but backfired just like Waco due to the incompetence of the agents. A secretary for the BATF also confirmed that on the first day when the first news crews arrived. She was very angry "we all knew something big was up because all the boys were out of the office", speaking of both the BATF and FBI offices, leaving the secretaries and other employees to take their chances. Sounds just like the BATF I know and loath.
The FBI's scenario has McViegh renting the truck in Texas one day and setting off the bomb the next morning. Impossible, you can't just load a couple of drums of fuel oil onto a truck of fertilizer and call it a bomb. It has to be well mixed and the proper mixture is a very dry one, very time consuming to mix. Even if he had rented a portable cement mixer, and they never claimed he did, it would still have been a horrendous job for one man to mix tons of it over night, he had to have help.