Three unidentified objects shot down by US fighter jets since Friday may turn out to be balloons connected to “benign” commercial or research efforts, a White House official said on Tuesday.

The US has not found any evidence to connect the objects to China’s balloon surveillance program nor to any other country’s spy program, national security council spokesperson, John Kirby, told reporters.

“We haven’t seen any indication or anything that points specifically to the idea that these three objects were part of the [People’s Republic of China’s] spying program, or that they were definitively involved in external intelligence collection efforts,” he said.

Instead, a “leading explanation” may be that the objects were operated privately for commercial or research purposes, Kirby said, though no one has stepped forward to claim ownership.

The unidentified object shot down by a US fighter jet over northern Canada on Saturday was a “small, metallic balloon with a tethered payload below it”, according to a Pentagon memo to US lawmakers obtained by CNN.

The details about the mysterious object – one of three shot down by the US in an eight-day period after a suspected Chinese spy balloon traversed the US mainland and was brought down over the Atlantic Ocean on 4 February – come amid growing frustration over a lack of information from Joe Biden’s presidential administration.

The object shot down over Alaska on Friday was the “size of a small car”, the memo states. It also warns against drawing hasty conclusions: “It should not be assumed that the events of the past few days are connected.”

The object shot down over Lake Huron on Sunday was previously described as an “octagonal structure” with strings attached to it.

“These objects did not closely resemble and were much smaller than the PRC balloon, and we will not definitively characterize them until we can recover the debris, which we are working on,” a national security council spokesperson told CNN.

Also on Tuesday, Gen Mark Milley, the US’s highest ranking military official, acknowledged at a briefing in Brussels that the first attempt to shoot down the object over Lake Huron missed.

The first missile “landed harmlessly” in the water while a second missile successfully downed the object, Milley said.

“We’re very, very careful to make sure that those shots are in fact safe,” Milley said, according to the Associated Press. “And that’s the guidance from the president. Shoot it down, but make sure we minimize collateral damage and we preserve the safety of the American people.”

US senators received a classified briefing about the objects on Tuesday. The briefers included an assistant secretary of defense for homeland defense and hemispheric affairs as well as Gen Glen VanHerck, commander of the North American Aerospace Defense Command, or Norad, according to Politico.

The information vacuum has fueled speculation about the origins of the unidentified flying objects detected and shot down over Deadhorse, Alaska; Yukon, Canada; and Lake Huron, Michigan, since Friday.

At a press briefing on Monday, administration officials defended Biden’s decision to shoot down the objects and also attempted to bat back at least one strand of the conspiracy theorizing inspired by the spate of unidentified flying objects, or UFOs.

“I know there have been questions and concerns about this, but there is no – again, no – indication of aliens or extraterrestrial activity with these recent takedowns,” the White House press secretary, Karine Jean-Pierre, said at a briefing on Monday. “I wanted to make sure that the American people knew that – all of you knew that – and it was important for us to say that from here because we’ve been hearing a lot about it.”

Pressed on whether political pressure played into the decision to shoot down the objects, Kirby said: “There were very good reasons to do it … These were decisions based purely and simply on what was in the best interests of the American people.”

The three objects shot down during the three-day period beginning Friday did not pose any threat to people on the ground, Kirby said, and were not sending communication signals.

However, their altitude – significantly lower than the Chinese balloon from earlier – meant that they did pose a “very real potential risk to civilian air traffic”.