President Trump knows one thing for sure about the sorry state of the Lincoln Memorial Reflecting Pool: It’s not his fault.

Speaking to reporters on Monday, Mr. Trump said the enormous blooms of green algae and the peeling polyurethane had nothing to do with the rushed $16.4 million makeover he had ordered. Instead, he laid out in vague terms a series of events that he claimed were responsible for the highly embarrassing breakdown of a Washington landmark.

To hear Mr. Trump tell it, the renovation was going perfectly until vandals armed with knives and fertilizer set their sights on the century-old pool, which stretches between the Washington Monument and the Lincoln Memorial.

“They put, somebody said, fertilizer in the water,” Mr. Trump said. “If you put fertilizer in the water, you get algae. But somebody said they might have put fertilizer. They did something to create the algae.”

Of the large strips of polyurethane spotted floating in the water, Mr. Trump said somebody had sliced the pool with “probably a box cutter or a knife of some kind.”

“I can’t help it if somebody goes in with a knife and starts hacking it up,” he said.

Katie Martin, a spokeswoman for the Interior Department, which includes the National Park Service, said in an email that five people had been arrested on charges of vandalism, and five others had been issued citations.

The names of those arrested and cited were not released, so it was impossible to know if they had actually been accused of dumping fertilizer or slashing the pool. But David Carter Hearn, 67, a cyclist and three-time Olympian as a canoeist, said he was arrested on Friday and charged with destroying government property, which he denies.

He said he stopped at the site on Friday just to have a look, then reached down to touch a strip of peeling blue paint mixed with the algae.

“I was just a curious, concerned citizen,” he said in an interview with The New York Times. “I guess I was there at the wrong place, wrong time.”

Aside from his claims of vandalism, the president also lashed out on Monday at former President Barack Obama for not properly fixing the Reflecting Pool.

Between 2010 and 2012, the Obama administration spent more than $35 million trying to solve those problems. It failed. The pool was matted with green algae within a month of its reopening. It still leaks 16 million gallons of water a year, which the National Park Service must pay to replace.

“Barack Hussein Obama, have you ever heard of him?” Mr. Trump snapped at reporters.

The Reflecting Pool has been troubled for decades by leaks and algae blooms, but Mr. Trump pledged to fix those problems ahead of the July 4 holiday, even as some experts warned a quick makeover would not fix the pool’s longer-term issues. Over the weekend, the president described issues with the project so severe that the pool would likely need to be partly drained for “necessary repairs.”

The Trump administration awarded two no-bid contracts for the Reflecting Pool renovations, bypassing the legally required process of seeking competitive bids because of what it called an urgent need to complete the project by July 4.

The first $14.7 million contract went to a Virginia-based company called Atlantic Industrial Coatings to spread the waterproofing sealant in the “American flag blue” shade. The second $1.7 million contract went to Ohio-based Greenwater Services to install a new water-treatment system.

In a post on its website on Sunday, Atlantic Industrial Coatings defended its work, saying the areas with peeling sealant were “a very small part of the massive 7-acre project.” It added that the pool would be drained for further repairs soon.