German prosecutors began an investigation into suspected negligence by Bayer yesterday, four weeks after the drugs and chemicals company withdrew its controversial anti-cholesterol drug Baycol.

The investigation began as Bayer came close to completing its long-awaited deal to buy Aventis's crop sciences business for 7.25bn ( pounds 4.5bn).

Prosecutors in Cologne want to know whether Bayer withheld information about side effects from patients in the weeks running up to the recall of Baycol, known as Lipobay in Britain. Public prosecutor Jurgen Kapischke said the inquiry could be widened to take in negligent bodily harm or manslaughter, although there were no grounds for this yet: For the time being we do not have sufficient grounds (to suspect) that people have died in Germany from Lipobay.'

Mr Kapischke said four complaints had been made against Bayer, including two from people who had taken the drug.

The prosecutor said the inquiry began several weeks ago in order to establish whether there was a case to answer. The National Institute for Drugs Manufacture and Prescription has handed over 31 files to prosecutors, mainly in English.

The failure of Baycol triggered a collapse in Bayer's share price last month. It forced the company, best known for inventing aspirin, to consider selling its drugs arm or giving up control of the business in a joint venture.

A spokesman at the firm's Leverkusen head office played down the investigation yesterday, saying there were nearly no facts given' by the state prosecutor: It's just an investigation into whether there's any need for further action. It's the lightest form of action the prosecutor's office can take.'

In a statement, Bayer said it had acted responsibly and in the interest of patient health and safety at all times', adding that the company would co-operate with the prosecutor. '

Bayer is nearing a deal to buy the agrochemicals division of Franco-German group Aventis, with an announcement likely within two weeks. This would double the size of the firm's agricultural division, propelling it to second place in world rankings behind Switzerland's Syngenta.

Speculation is growing that once that acquisition is complete, Bayer could concentrate on selling its healthcare operations. Until recently, the firm opted out of the rapid consolidation of the drugs industry saying it was big enough to prosper alone.

Possible buyers for the drugs division include Glaxo SmithKline, Sanofi, Bristol-Myers Squibb and Aventis. Swiss group Roche last week denied making an offer but is tipped as a candidate.

Some analystshave questioned the attraction of the business. One said: Bayer hasn't got a lot else - Baycol was the cornerstone of its profit growth.'

The next significant product in Bayer's drugs pipeline is vardenafil, a drug for erectile dysfunction to rival Viagra, which has proven promising in third-stage clinical trials.

Baycol, which was taken by 7m people, has been linked with 52 deaths. The most serious side effect is rhamdomyolysis, a muscle wasting condition which can lead to the failure of internal organs.