11 am: This post has been updated.

BP's ruptured well in the Gulf hasn't gushed crude for more than a month now, but engineers are still proceeding with caution. The additional consideration is delaying the final fix, which entails using a relief well to intercept the damaged well and pump mud and cement into the system — a procedure called a “bottom kill.” If those steps go well, it could mean that the Gulf well has at last been killed.

The Wall Street Journal, which called the delay “an unexpected complication,” also pointed out that the official leading the spill response, Adm. Thad Allen, had previously said the decision would be made on Tuesday or Wednesday of this week. He now says he doesn't know how long it will be until the decision is made to proceed with finishing the relief well. Allen told reporters on Wednesday that the delay in decisionmaking is "nothing more than an overabundance of caution in being responsible and doing our jobs."

At issue right now is how to be sure that pressure from the bottom kill won't further damage the well. One option is to replace the failed blowout preventer, known in the industry as a BOP, with a new one that is expected to be stronger than the failed device -- and to do this before the bottom kill procedure has occurred. Previously, the plan was to replace the blowout preventer after the bottom kill procedure. Allen has noted that the Justice Department would likely want the original equipment as evidence in its investigation of the disaster.

In any case, Allen has ordered BP to bring in another blowout preventer to have on hand, ready to make the switch. According to Bloomberg, this blowout preventer, supposed to be stronger than the original one, “would be taken from a second relief well.” (Operations on the second relief well have been suspended since last Monday.)

But if you think back to July, you may recall that the blowout preventers on BP’s relief wells also had problems — a fact tucked into a 29-page Interior Department memo:

It is clear that the apparent performance problem with the Deepwater Horizon’s BOP is not an isolated incident. Performance problems have also been identified in recent weeks with the BOPs on the relief wells that BP is drilling.

According to that memo, the problems were detected and fixed, but given that both Adm. Allen and the media have been referring to it as a “new” blowout preventer, it’s worth revisiting what we know about it: It’s not foolproof either.

Update, 8/19: A federal official has just told the Associated Press that the final plugging of BP's well won't happen until after Labor Day, which is September 6. He said the decision was made last night, just hours after Adm. Allen had refused to give reporters a timeline for killing the well. The well was originally expected to be killed by mid-August.