In an interview on "Fox and Friends", Grassley observed that, "There's three years we don't have any information for".
"And then in regard to where we did get the information, it was so heavily redacted that it was hard to make sense out of it", he added.
The information gap led some Republican senators to compose a letter to the director of the Secret Service, James Murray, over a lack of transparency.
Grassley believes that the oversight powers granted to Congress in the US Constitution should prevent documents from being redacted.
"It's just like pulling teeth to get answers from the executive branch, particularly on this subject", he said.
Hunter Biden's proximity to his father, US President Joe Biden, while the latter was vice president has dragged him into Grassley's circle of suspicion.
"We know that [Hunter Biden] travelled sometimes in an airplane with his father. I can't make any accusations against the president of the United States, but I think we have a right to know what the son was doing in those instances", he said.
When the Iowa senator was asked if he thought that the Secret Service was covering for Hunter Biden, he demurred from asserting it as truth.
"At this point, you can't draw any other conclusions", Grassley suggested, adding, "It's very suspicious".
The lack of media attention on the subject appeared to trouble the Republican as he said that the press needs to act like "policemen" and hold politicians accountable. He cited the importance of the First Amendment in the matter.
"That's what the First Amendment, freedom of speech and press, is all about – to make sure that government doesn't interfere".
Hunter Biden's business dealings in China, Ukraine, and globally earlier prompted a federal investigation. While his father was vice president, the younger Biden reportedly earned a $10 million a year fee from a Chinese billionaire for introductions.
The US president has denied any connection to his son's business dealings.