Donald Trump and the Department of the Interior are facing growing pressure to release photo and video evidence substantiating their claims of sabotage at the Lincoln Memorial reflecting pool in Washington.

The $14.7m renovation of the landmark has descended into a farce of algae blooms, peeling paint and dead ducks just days before the US’s 250th anniversary celebrations. Crews have been seen erecting fencing near the area.

On Wednesday the US president posted an image that purportedly showed the pool before it was recently refilled with water. “This is the hard rubber surface – No Paint – Before the Vandals cut and pulled it apart!” he wrote on his Truth Social platform.

In a previous post, Trump had alleged without providing evidence that a “350-foot gash” had been deliberately carved into the pool’s lining. “It was purposefully and criminally done and somebody had to work very hard, probably in the dark of night, to create such a condition.”

The president added that the interior department would make public photographs and video to prove his point. But as of Wednesday afternoon, no such evidence had materialised. Further doubt was cast on Trump’s claims when the New York Times said it obtained government documents that gave no indication peeling paint and algae blooms were caused intentionally.

On Wednesday, the Democratic senator Richard Blumenthal sent a letter to the secretary of the interior, Doug Burgum, and National Park Service acting director, Jessica Bowron, requesting documents related to the project, which he said has been “marked by blatant corruption, a shocking lack of transparency, disregard for legal requirements, and apparent incompetence”.

“The American people deserve to know how this occurred and what other issues plague the work NPS is currently undertaking in our nation’s capital,” the senator wrote.

Trump had pledged to transform the century-old, 2,028ft reflecting pool before the semiquincentennial festivities, draining it and having its bottom coated in a colour he personally selected and dubbed “American flag blue”. He declared the project complete on 6 June, promising the pool would become a gleaming expanse along the National Mall.

But a vivid green algae bloom soon clouded the water, obscuring the new blue lining, and pieces of the coating were observed peeling away. A section of liner roughly four square feet in area was seen partially floating on Friday.

A dead duckling was found floating in the water on Sunday and the bodies of two more birds – a juvenile and an adult – were recovered from a pond about 250ft away from the pool, according to the Center for Biological Diversity, an environmental group. The Center has called on the US Fish and Wildlife Service to launch an immediate investigation and enforce the Migratory Bird Treaty Act.

“Wasting taxpayer money turning the reflecting pool into a giant duck death trap just in time for America’s 250th birthday party is as Trump as it gets,” said Tara Zuardo, a senior campaigner at the Center. “Cruel, stupid and selfish.”

Workers have been attempting to combat the algae by pouring hydrogen peroxide into the pool – a chemical that can itself act as a paint remover. The interior department has also said it is using “high-tech nanobubble ozone technology” to effectively cut off the algae’s food supply.

Experts have noted that the dark lining is likely to have exacerbated the problem, absorbing more sunlight than a lighter surface and raising the surrounding water temperature, creating conditions in which algae thrive.

Atlantic Industrial Coatings, the Virginia-based company in charge of the renovation, insisted the affected areas represented “a very small part of the massive seven-acre project, and do not indicate a failure of the liner”. It said it expected to carry out repairs once the pool is drained again, under the terms of its warranty. Trump awarded a no-bid contract to the company, which he said had previously done work on swimming pools at one of his golf clubs.

DC Water has issued a permit for the pool to be drained. Trump confirmed that “some of the water” would be removed “either immediately before or after the Fourth of July, to do the permanent repair”, though the scale, scope and cost of those repairs remained unclear.

The affair has drawn national guard members and park police to patrol the deck around the pool since the weekend, when Trump first insisted that vandals were to blame. Fencing was also being erected around the site on Tuesday evening.

An interior department spokesperson, Katie Martin, told the Associated Press news agency that the fencing “was always set to be installed ahead of the Fourth of July”, but that it had been brought forward due to what she described as an “increase in vandalism by leftist activists” – a claim for which she offered no evidence.

Trump has repeatedly threatened severe consequences for anyone found to have harmed the pool. “Please remember that there is a 10-year prison sentence for the destruction, or even the attempted destruction, of such things – which will be fully enforced!” he wrote in one post.

Trump has asserted that six people have been arrested for allegedly damaging the pool. One has been confirmed: former Olympic canoe racer David Hearn, 67, of Bethesda, Maryland, who has claimed he reached into the pool merely to examine the peeling coating and was detained by national guard troops and park police for five hours.

“This was a botched job from the beginning,” said Paul Strauss, the senior US shadow senator for the District of Columbia, adding: “This is what happens when instead of going through the proper government contracting process you get your buddies from down the street at Mar-a-Lago to do a job quickly.”