According to the Coast Guard's assesment, made public on Monday, US intelligence gaps made it impossible to determine whether Dubai-based DP World might support terrorist operations.
Officials of President Bush's administration say that that those concerns have been addressed and resolved.
However, Republicans and Democrats have seized on the Coast Guard report, released by Susan Collins, a Republican senator, at a homeland security committee hearing, to launch a new round of criticisms.
Collins, who chairs the committee, said she was "more convinced than ever that the process was truly flawed".
She said: "I can only conclude that there was a rush to judgment, that there wasn't the kind of painstaking, thorough analysis that needed to be done, despite serious questions being raised and despite the involvement of a wide variety of agencies."
Democrat senators were equally critical.
Charles Schumer, a Democrat from New York, said: "If this isn't a smoking gun, it shows that there may be one undetected."
Critics say a multi-agency panel boke the law when it signed off on the ports deal without doing a 45-day investigation into security implications.
"If this isn't a smoking gun, it shows that there may be one undetected"
Collins has now released an unclassified excerpt of the Coast Guard intelligence assessment carried out before the government approved the DP World takeover.
The half-page excerpt said: "There are many intelligence gaps, concerning the potential for DPW [DP World] or P&O assets to support terrorist operations, that precludes an overall threat assessment" of the potential merger.
"The breadth of the intelligence gaps also infer potential unknown threats against a large number of potential vulnerabilities."
The document raised questions about the security of the companies' operations, the backgrounds of people working for them, and whether other foreign countries influenced operations that affect security.
Under the agreement congressional Republican leaders negotiated over the weekend, the Bush administration agreed to the company's own request for a 45-day national security review of its business deal.
But a bipartisan group of senators introduced a bill that would delay the deal and give Congress an opportunity to block the takeover.
Meanwhile, in a separate development Democrat senators have introduced legislation that would prohibit companies owned by foreign governments from controlling operations at US ports.