Critical Americans Speak in Whispers
Now-famous retired CIA guy Robert Baer is one who's always suspected Iranian involvement, with little if any help from Libya. Alan Gerson and Jerry Adler's 2001 book The Price of Terror mentions a meeting Baer had with PA103 families leader Victoria Cummock, in which he expressed his concern that Iran's role was minimized. Cummock, who lost her husband in the boombing, told the book:
"He voiced what Paul Hudson and I had been saying to each other all thhrough the end of 1990 and into 1991. We were getting the State Department briefings, and we both noticed there was less and less in them about Syria and the Iranians. Then we stopped hearing anything about them." [p 91]This was all just "intelligence," as Marquise would call it, and "whispers" per Gerson and Adler, easily trumped by "evidence." This would seem so, but the intelligence was good and had some evidence of its own, and the evidence that replaced it pointing to Libya (alone or at all) is rotten down the line.
John Biewen and/or Ian ferguson wrote in 2000:
"Nonetheless, Cannistraro says he is persuaded that the Libyans are guilty. He says Qadhafi's government was in touch with the PFLP-GC and had, in fact, subsidized the group. But Cannistraro says he's convinced Qadhafi's men were hired to finish the Pan Am bombing only after the West German police broke up the PFLP-GC's operation. "I do think the Libyans carried it out. But I believe more it was a hand-off from the PFLP-GC after their own operational cell was compromised."
So, who conspired to bomb Pan Am 103? Iran, Syria, the PFLP-GC, or the Libyans? Cannistraro's answer is: maybe all of the above." 
Alan Gerson and Jerry Adler, The Price of Terror, 2001:
"The French claimed to have information that both the UTA  and Pan Am  bombings were decided on at a meeting in Tripoli in September 1988. If true, that would tend to exonerate Jibril and Iran in the Pan Am bombing, because it implies Libya was planning the attack even before the PFLP-GC cell in Germany was rounded up; Jibril would have had no reason to hand off the job to the Libyans until after the arrests, in October. But American investigators never confirmed this September meeting." [p 98]
See also: Libya's "Admissions of Guilt" including a strong dose of Iranian initiative and the same motive. Cited there, an alleged confession by Gaddafi to an American journalist saying in part:
"We knew it would be comparable retaliation for the Iranian Airbus, but we were not told what the specific objective was [...] If we had initiated the plot, we would have made sure the accusing finger was pointed in the other direction and we would have picked Cyprus, not Malta, where some of the organization was done. The others picked Malta presumably to frame us." - Moammar Gaddafi, attributed by Arnaud de Borchgrave, Summer 1993 
Biewen and Ferguson:
"But the Lockerbie indictment of 1991 pointed the finger only at the Libyans. In a comment shortly after the indictment, President Bush explicitly exonerated Damascus. "The Syrians took a bum rap on this," Bush said."International Herald Tribune, Jan 13 1994
[see also Gerson and Adler, p 92]
"Libya Still Only Suspect in Bombing
There is no evidence that any country other than Libya was involved in the bombing of a Pan Am jumbo jet over Scotland in 1988, but the inquiry into the matter remains open, Prime Minister John Major said Wednesday.
Mr. Major was asked in the House of Commons about reports suggesting that Syria and Iran might have been involved in the bombing, which killed all 259 people on board the New York-bound flight and 11 people on the ground in Lockerbie, Scotland.
Britain and the United States have named two Libyans as suspects in the bombing, and the United Nations has imposed sanctions against Libya because it has refused to extradite the suspects." 
"Robert Mueller, U.S. Assistant Attorney General] We have no evidence to implicate another country in this disaster.
[Douglas Hurd, British Foreign Secretary] I understand the investigation has revealed no evidence to support suggestions of involvement by any other countries." 
6 March 1995: Mr. Douglas Hogg: "The Lockerbie investigators have given exhaustive consideration to all information relevant to the Lockerbie bombing. The possible involvement by nationals of a number of countries has been very closely investigated. Despite the unprecedented scale of the investigation, the available evidence does not support charges against the nationals of any country besides Libya. But the investigation remains open and any relevant new information will be considered." ---
 Biewen, John and Ian Ferguson. "Mass Murder Over Scotland." Shadow over Lockerie series. American Radio Works, 2000.